/*
* RT-Mutexes: simple blocking mutual exclusion locks with PI support
*
* started by Ingo Molnar and Thomas Gleixner.
*
* Copyright (C) 2004-2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
* Copyright (C) 2005-2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
* Copyright (C) 2005 Kihon Technologies Inc., Steven Rostedt
* Copyright (C) 2006 Esben Nielsen
*
* See Documentation/locking/rt-mutex-design.txt for details.
*/
#include <linux/spinlock.h>
#include <linux/export.h>
#include <linux/sched.h>
#include <linux/sched/rt.h>
#include <linux/sched/deadline.h>
#include <linux/timer.h>
#include "rtmutex_common.h"
/*
* lock->owner state tracking:
*
* lock->owner holds the task_struct pointer of the owner. Bit 0
* is used to keep track of the "lock has waiters" state.
*
* owner bit0
* NULL 0 lock is free (fast acquire possible)
* NULL 1 lock is free and has waiters and the top waiter
* is going to take the lock*
* taskpointer 0 lock is held (fast release possible)
* taskpointer 1 lock is held and has waiters**
*
* The fast atomic compare exchange based acquire and release is only
* possible when bit 0 of lock->owner is 0.
*
* (*) It also can be a transitional state when grabbing the lock
* with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
* we need to set the bit0 before looking at the lock, and the owner may be
* NULL in this small time, hence this can be a transitional state.
*
* (**) There is a small time when bit 0 is set but there are no
* waiters. This can happen when grabbing the lock in the slow path.
* To prevent a cmpxchg of the owner releasing the lock, we need to
* set this bit before looking at the lock.
*/
static void
rt_mutex_set_owner(struct rt_mutex *lock, struct task_struct *owner)
{
unsigned long val = (unsigned long)owner;
if (rt_mutex_has_waiters(lock))
val |= RT_MUTEX_HAS_WAITERS;
lock->owner = (struct task_struct *)val;
}
static inline void clear_rt_mutex_waiters(struct rt_mutex *lock)
{
lock->owner = (struct task_struct *)
((unsigned long)lock->owner & ~RT_MUTEX_HAS_WAITERS);
}
static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
{
if (!rt_mutex_has_waiters(lock))
clear_rt_mutex_waiters(lock);
}
/*
* We can speed up the acquire/release, if the architecture
* supports cmpxchg and if there's no debugging state to be set up
*/
#if defined(__HAVE_ARCH_CMPXCHG) && !defined(CONFIG_DEBUG_RT_MUTEXES)
# define rt_mutex_cmpxchg(l,c,n) (cmpxchg(&l->owner, c, n) == c)
static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
{
unsigned long owner, *p = (unsigned long *) &lock->owner;
do {
owner = *p;
} while (cmpxchg(p, owner, owner | RT_MUTEX_HAS_WAITERS) != owner);
}
/*
* Safe fastpath aware unlock:
* 1) Clear the waiters bit
* 2) Drop lock->wait_lock
* 3) Try to unlock the lock with cmpxchg
*/
static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
__releases(lock->wait_lock)
{
struct task_struct *owner = rt_mutex_owner(lock);
clear_rt_mutex_waiters(lock);
raw_spin_unlock(&lock->wait_lock);
/*
* If a new waiter comes in between the unlock and the cmpxchg
* we have two situations:
*
* unlock(wait_lock);
* lock(wait_lock);
* cmpxchg(p, owner, 0) == owner
* mark_rt_mutex_waiters(lock);
* acquire(lock);
* or:
*
* unlock(wait_lock);
* lock(wait_lock);
* mark_rt_mutex_waiters(lock);
*
* cmpxchg(p, owner, 0) != owner
* enqueue_waiter();
* unlock(wait_lock);
* lock(wait_lock);
* wake waiter();
* unlock(wait_lock);
* lock(wait_lock);
* acquire(lock);
*/
return rt_mutex_cmpxchg(lock, owner, NULL);
}
#else
# define rt_mutex_cmpxchg(l,c,n) (0)
static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
{
lock->owner = (struct task_struct *)
((unsigned long)lock->owner | RT_MUTEX_HAS_WAITERS);
}
/*
* Simple slow path only version: lock->owner is protected by lock->wait_lock.
*/
static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
__releases(lock->wait_lock)
{
lock->owner = NULL;
raw_spin_unlock(&lock->wait_lock);
return true;
}
#endif
static inline int
rt_mutex_waiter_less(struct rt_mutex_waiter *left,
struct rt_mutex_waiter *right)
{
if (left->prio < right->prio)
return 1;
/*
* If both waiters have dl_prio(), we check the deadlines of the
* associated tasks.
* If left waiter has a dl_prio(), and we didn't return 1 above,
* then right waiter has a dl_prio() too.
*/
if (dl_prio(left->prio))
return (left->task->dl.deadline < right->task->dl.deadline);
return 0;
}
static void
rt_mutex_enqueue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
{
struct rb_node **link = &lock->waiters.rb_node;
struct rb_node *parent = NULL;
struct rt_mutex_waiter *entry;
int leftmost = 1;
while (*link) {
parent = *link;
entry = rb_entry(parent, struct rt_mutex_waiter, tree_entry);
if (rt_mutex_waiter_less(waiter, entry)) {
link = &parent->rb_left;
} else {
link = &parent->rb_right;
leftmost = 0;
}
}
if (leftmost)
lock->waiters_leftmost = &waiter->tree_entry;
rb_link_node(&waiter->tree_entry, parent, link);
rb_insert_color(&waiter->tree_entry, &lock->waiters);
}
static void
rt_mutex_dequeue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
{
if (RB_EMPTY_NODE(&waiter->tree_entry))
return;
if (lock->waiters_leftmost == &waiter->tree_entry)
lock->waiters_leftmost = rb_next(&waiter->tree_entry);
rb_erase(&waiter->tree_entry, &lock->waiters);
RB_CLEAR_NODE(&waiter->tree_entry);
}
static void
rt_mutex_enqueue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
{
struct rb_node **link = &task->pi_waiters.rb_node;
struct rb_node *parent = NULL;
struct rt_mutex_waiter *entry;
int leftmost = 1;
while (*link) {
parent = *link;
entry = rb_entry(parent, struct rt_mutex_waiter, pi_tree_entry);
if (rt_mutex_waiter_less(waiter, entry)) {
link = &parent->rb_left;
} else {
link = &parent->rb_right;
leftmost = 0;
}
}
if (leftmost)
task->pi_waiters_leftmost = &waiter->pi_tree_entry;
rb_link_node(&waiter->pi_tree_entry, parent, link);
rb_insert_color(&waiter->pi_tree_entry, &task->pi_waiters);
}
static void
rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
{
if (RB_EMPTY_NODE(&waiter->pi_tree_entry))
return;
if (task->pi_waiters_leftmost == &waiter->pi_tree_entry)
task->pi_waiters_leftmost = rb_next(&waiter->pi_tree_entry);
rb_erase(&waiter->pi_tree_entry, &task->pi_waiters);
RB_CLEAR_NODE(&waiter->pi_tree_entry);
}
/*
* Calculate task priority from the waiter tree priority
*
* Return task->normal_prio when the waiter tree is empty or when
* the waiter is not allowed to do priority boosting
*/
int rt_mutex_getprio(struct task_struct *task)
{
if (likely(!task_has_pi_waiters(task)))
return task->normal_prio;
return min(task_top_pi_waiter(task)->prio,
task->normal_prio);
}
struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
{
if (likely(!task_has_pi_waiters(task)))
return NULL;
return task_top_pi_waiter(task)->task;
}
/*
* Called by sched_setscheduler() to check whether the priority change
* is overruled by a possible priority boosting.
*/
int rt_mutex_check_prio(struct task_struct *task, int newprio)
{
if (!task_has_pi_waiters(task))
return 0;
return task_top_pi_waiter(task)->task->prio <= newprio;
}
/*
* Adjust the priority of a task, after its pi_waiters got modified.
*
* This can be both boosting and unboosting. task->pi_lock must be held.
*/
static void __rt_mutex_adjust_prio(struct task_struct *task)
{
int prio = rt_mutex_getprio(task);
if (task->prio != prio || dl_prio(prio))
rt_mutex_setprio(task, prio);
}
/*
* Adjust task priority (undo boosting). Called from the exit path of
* rt_mutex_slowunlock() and rt_mutex_slowlock().
*
* (Note: We do this outside of the protection of lock->wait_lock to
* allow the lock to be taken while or before we readjust the priority
* of task. We do not use the spin_xx_mutex() variants here as we are
* outside of the debug path.)
*/
static void rt_mutex_adjust_prio(struct task_struct *task)
{
unsigned long flags;
raw_spin_lock_irqsave(&task->pi_lock, flags);
__rt_mutex_adjust_prio(task);
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
}
/*
* Deadlock detection is conditional:
*
* If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
* if the detect argument is == RT_MUTEX_FULL_CHAINWALK.
*
* If CONFIG_DEBUG_RT_MUTEXES=y, deadlock detection is always
* conducted independent of the detect argument.
*
* If the waiter argument is NULL this indicates the deboost path and
* deadlock detection is disabled independent of the detect argument
* and the config settings.
*/
static bool rt_mutex_cond_detect_deadlock(struct rt_mutex_waiter *waiter,
enum rtmutex_chainwalk chwalk)
{
/*
* This is just a wrapper function for the following call,
* because debug_rt_mutex_detect_deadlock() smells like a magic
* debug feature and I wanted to keep the cond function in the
* main source file along with the comments instead of having
* two of the same in the headers.
*/
return debug_rt_mutex_detect_deadlock(waiter, chwalk);
}
/*
* Max number of times we'll walk the boosting chain:
*/
int max_lock_depth = 1024;
static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
{
return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
}
/*
* Adjust the priority chain. Also used for deadlock detection.
* Decreases task's usage by one - may thus free the task.
*
* @task: the task owning the mutex (owner) for which a chain walk is
* probably needed
* @deadlock_detect: do we have to carry out deadlock detection?
* @orig_lock: the mutex (can be NULL if we are walking the chain to recheck
* things for a task that has just got its priority adjusted, and
* is waiting on a mutex)
* @next_lock: the mutex on which the owner of @orig_lock was blocked before
* we dropped its pi_lock. Is never dereferenced, only used for
* comparison to detect lock chain changes.
* @orig_waiter: rt_mutex_waiter struct for the task that has just donated
* its priority to the mutex owner (can be NULL in the case
* depicted above or if the top waiter is gone away and we are
* actually deboosting the owner)
* @top_task: the current top waiter
*
* Returns 0 or -EDEADLK.
*
* Chain walk basics and protection scope
*
* [R] refcount on task
* [P] task->pi_lock held
* [L] rtmutex->wait_lock held
*
* Step Description Protected by
* function arguments:
* @task [R]
* @orig_lock if != NULL @top_task is blocked on it
* @next_lock Unprotected. Cannot be
* dereferenced. Only used for
* comparison.
* @orig_waiter if != NULL @top_task is blocked on it
* @top_task current, or in case of proxy
* locking protected by calling
* code
* again:
* loop_sanity_check();
* retry:
* [1] lock(task->pi_lock); [R] acquire [P]
* [2] waiter = task->pi_blocked_on; [P]
* [3] check_exit_conditions_1(); [P]
* [4] lock = waiter->lock; [P]
* [5] if (!try_lock(lock->wait_lock)) { [P] try to acquire [L]
* unlock(task->pi_lock); release [P]
* goto retry;
* }
* [6] check_exit_conditions_2(); [P] + [L]
* [7] requeue_lock_waiter(lock, waiter); [P] + [L]
* [8] unlock(task->pi_lock); release [P]
* put_task_struct(task); release [R]
* [9] check_exit_conditions_3(); [L]
* [10] task = owner(lock); [L]
* get_task_struct(task); [L] acquire [R]
* lock(task->pi_lock); [L] acquire [P]
* [11] requeue_pi_waiter(tsk, waiters(lock));[P] + [L]
* [12] check_exit_conditions_4(); [P] + [L]
* [13] unlock(task->pi_lock); release [P]
* unlock(lock->wait_lock); release [L]
* goto again;
*/
static int rt_mutex_adjust_prio_chain(struct task_struct *task,
enum rtmutex_chainwalk chwalk,
struct rt_mutex *orig_lock,
struct rt_mutex *next_lock,
struct rt_mutex_waiter *orig_waiter,
struct task_struct *top_task)
{
struct rt_mutex_waiter *waiter, *top_waiter = orig_waiter;
struct rt_mutex_waiter *prerequeue_top_waiter;
int ret = 0, depth = 0;
struct rt_mutex *lock;
bool detect_deadlock;
unsigned long flags;
bool requeue = true;
detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter, chwalk);
/*
* The (de)boosting is a step by step approach with a lot of
* pitfalls. We want this to be preemptible and we want hold a
* maximum of two locks per step. So we have to check
* carefully whether things change under us.
*/
again:
/*
* We limit the lock chain length for each invocation.
*/
if (++depth > max_lock_depth) {
static int prev_max;
/*
* Print this only once. If the admin changes the limit,
* print a new message when reaching the limit again.
*/
if (prev_max != max_lock_depth) {
prev_max = max_lock_depth;
printk(KERN_WARNING "Maximum lock depth %d reached "
"task: %s (%d)\n", max_lock_depth,
top_task->comm, task_pid_nr(top_task));
}
put_task_struct(task);
return -EDEADLK;
}
/*
* We are fully preemptible here and only hold the refcount on
* @task. So everything can have changed under us since the
* caller or our own code below (goto retry/again) dropped all
* locks.
*/
retry:
/*
* [1] Task cannot go away as we did a get_task() before !
*/
raw_spin_lock_irqsave(&task->pi_lock, flags);
/*
* [2] Get the waiter on which @task is blocked on.
*/
waiter = task->pi_blocked_on;
/*
* [3] check_exit_conditions_1() protected by task->pi_lock.
*/
/*
* Check whether the end of the boosting chain has been
* reached or the state of the chain has changed while we
* dropped the locks.
*/
if (!waiter)
goto out_unlock_pi;
/*
* Check the orig_waiter state. After we dropped the locks,
* the previous owner of the lock might have released the lock.
*/
if (orig_waiter && !rt_mutex_owner(orig_lock))
goto out_unlock_pi;
/*
* We dropped all locks after taking a refcount on @task, so
* the task might have moved on in the lock chain or even left
* the chain completely and blocks now on an unrelated lock or
* on @orig_lock.
*
* We stored the lock on which @task was blocked in @next_lock,
* so we can detect the chain change.
*/
if (next_lock != waiter->lock)
goto out_unlock_pi;
/*
* Drop out, when the task has no waiters. Note,
* top_waiter can be NULL, when we are in the deboosting
* mode!
*/
if (top_waiter) {
if (!task_has_pi_waiters(task))
goto out_unlock_pi;
/*
* If deadlock detection is off, we stop here if we
* are not the top pi waiter of the task. If deadlock
* detection is enabled we continue, but stop the
* requeueing in the chain walk.
*/
if (top_waiter != task_top_pi_waiter(task)) {
if (!detect_deadlock)
goto out_unlock_pi;
else
requeue = false;
}
}
/*
* If the waiter priority is the same as the task priority
* then there is no further priority adjustment necessary. If
* deadlock detection is off, we stop the chain walk. If its
* enabled we continue, but stop the requeueing in the chain
* walk.
*/
if (waiter->prio == task->prio) {
if (!detect_deadlock)
goto out_unlock_pi;
else
requeue = false;
}
/*
* [4] Get the next lock
*/
lock = waiter->lock;
/*
* [5] We need to trylock here as we are holding task->pi_lock,
* which is the reverse lock order versus the other rtmutex
* operations.
*/
if (!raw_spin_trylock(&lock->wait_lock)) {
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
cpu_relax();
goto retry;
}
/*
* [6] check_exit_conditions_2() protected by task->pi_lock and
* lock->wait_lock.
*
* Deadlock detection. If the lock is the same as the original
* lock which caused us to walk the lock chain or if the
* current lock is owned by the task which initiated the chain
* walk, we detected a deadlock.
*/
if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
debug_rt_mutex_deadlock(chwalk, orig_waiter, lock);
raw_spin_unlock(&lock->wait_lock);
ret = -EDEADLK;
goto out_unlock_pi;
}
/*
* If we just follow the lock chain for deadlock detection, no
* need to do all the requeue operations. To avoid a truckload
* of conditionals around the various places below, just do the
* minimum chain walk checks.
*/
if (!requeue) {
/*
* No requeue[7] here. Just release @task [8]
*/
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
put_task_struct(task);
/*
* [9] check_exit_conditions_3 protected by lock->wait_lock.
* If there is no owner of the lock, end of chain.
*/
if (!rt_mutex_owner(lock)) {
raw_spin_unlock(&lock->wait_lock);
return 0;
}
/* [10] Grab the next task, i.e. owner of @lock */
task = rt_mutex_owner(lock);
get_task_struct(task);
raw_spin_lock_irqsave(&task->pi_lock, flags);
/*
* No requeue [11] here. We just do deadlock detection.
*
* [12] Store whether owner is blocked
* itself. Decision is made after dropping the locks
*/
next_lock = task_blocked_on_lock(task);
/*
* Get the top waiter for the next iteration
*/
top_waiter = rt_mutex_top_waiter(lock);
/* [13] Drop locks */
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
raw_spin_unlock(&lock->wait_lock);
/* If owner is not blocked, end of chain. */
if (!next_lock)
goto out_put_task;
goto again;
}
/*
* Store the current top waiter before doing the requeue
* operation on @lock. We need it for the boost/deboost
* decision below.
*/
prerequeue_top_waiter = rt_mutex_top_waiter(lock);
/* [7] Requeue the waiter in the lock waiter list. */
rt_mutex_dequeue(lock, waiter);
waiter->prio = task->prio;
rt_mutex_enqueue(lock, waiter);
/* [8] Release the task */
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
put_task_struct(task);
/*
* [9] check_exit_conditions_3 protected by lock->wait_lock.
*
* We must abort the chain walk if there is no lock owner even
* in the dead lock detection case, as we have nothing to
* follow here. This is the end of the chain we are walking.
*/
if (!rt_mutex_owner(lock)) {
/*
* If the requeue [7] above changed the top waiter,
* then we need to wake the new top waiter up to try
* to get the lock.
*/
if (prerequeue_top_waiter != rt_mutex_top_waiter(lock))
wake_up_process(rt_mutex_top_waiter(lock)->task);
raw_spin_unlock(&lock->wait_lock);
return 0;
}
/* [10] Grab the next task, i.e. the owner of @lock */
task = rt_mutex_owner(lock);
get_task_struct(task);
raw_spin_lock_irqsave(&task->pi_lock, flags);
/* [11] requeue the pi waiters if necessary */
if (waiter == rt_mutex_top_waiter(lock)) {
/*
* The waiter became the new top (highest priority)
* waiter on the lock. Replace the previous top waiter
* in the owner tasks pi waiters list with this waiter
* and adjust the priority of the owner.
*/
rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
rt_mutex_enqueue_pi(task, waiter);
__rt_mutex_adjust_prio(task);
} else if (prerequeue_top_waiter == waiter) {
/*
* The waiter was the top waiter on the lock, but is
* no longer the top prority waiter. Replace waiter in
* the owner tasks pi waiters list with the new top
* (highest priority) waiter and adjust the priority
* of the owner.
* The new top waiter is stored in @waiter so that
* @waiter == @top_waiter evaluates to true below and
* we continue to deboost the rest of the chain.
*/
rt_mutex_dequeue_pi(task, waiter);
waiter = rt_mutex_top_waiter(lock);
rt_mutex_enqueue_pi(task, waiter);
__rt_mutex_adjust_prio(task);
} else {
/*
* Nothing changed. No need to do any priority
* adjustment.
*/
}
/*
* [12] check_exit_conditions_4() protected by task->pi_lock
* and lock->wait_lock. The actual decisions are made after we
* dropped the locks.
*
* Check whether the task which owns the current lock is pi
* blocked itself. If yes we store a pointer to the lock for
* the lock chain change detection above. After we dropped
* task->pi_lock next_lock cannot be dereferenced anymore.
*/
next_lock = task_blocked_on_lock(task);
/*
* Store the top waiter of @lock for the end of chain walk
* decision below.
*/
top_waiter = rt_mutex_top_waiter(lock);
/* [13] Drop the locks */
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
raw_spin_unlock(&lock->wait_lock);
/*
* Make the actual exit decisions [12], based on the stored
* values.
*
* We reached the end of the lock chain. Stop right here. No
* point to go back just to figure that out.
*/
if (!next_lock)
goto out_put_task;
/*
* If the current waiter is not the top waiter on the lock,
* then we can stop the chain walk here if we are not in full
* deadlock detection mode.
*/
if (!detect_deadlock && waiter != top_waiter)
goto out_put_task;
goto again;
out_unlock_pi:
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
out_put_task:
put_task_struct(task);
return ret;
}
/*
* Try to take an rt-mutex
*
* Must be called with lock->wait_lock held.
*
* @lock: The lock to be acquired.
* @task: The task which wants to acquire the lock
* @waiter: The waiter that is queued to the lock's wait list if the
* callsite called task_blocked_on_lock(), otherwise NULL
*/
static int try_to_take_rt_mutex(struct rt_mutex *lock, struct task_struct *task,
struct rt_mutex_waiter *waiter)
{
unsigned long flags;
/*
* Before testing whether we can acquire @lock, we set the
* RT_MUTEX_HAS_WAITERS bit in @lock->owner. This forces all
* other tasks which try to modify @lock into the slow path
* and they serialize on @lock->wait_lock.
*
* The RT_MUTEX_HAS_WAITERS bit can have a transitional state
* as explained at the top of this file if and only if:
*
* - There is a lock owner. The caller must fixup the
* transient state if it does a trylock or leaves the lock
* function due to a signal or timeout.
*
* - @task acquires the lock and there are no other
* waiters. This is undone in rt_mutex_set_owner(@task) at
* the end of this function.
*/
mark_rt_mutex_waiters(lock);
/*
* If @lock has an owner, give up.
*/
if (rt_mutex_owner(lock))
return 0;
/*
* If @waiter != NULL, @task has already enqueued the waiter
* into @lock waiter list. If @waiter == NULL then this is a
* trylock attempt.
*/
if (waiter) {
/*
* If waiter is not the highest priority waiter of
* @lock, give up.
*/
if (waiter != rt_mutex_top_waiter(lock))
return 0;
/*
* We can acquire the lock. Remove the waiter from the
* lock waiters list.
*/
rt_mutex_dequeue(lock, waiter);
} else {
/*
* If the lock has waiters already we check whether @task is
* eligible to take over the lock.
*
* If there are no other waiters, @task can acquire
* the lock. @task->pi_blocked_on is NULL, so it does
* not need to be dequeued.
*/
if (rt_mutex_has_waiters(lock)) {
/*
* If @task->prio is greater than or equal to
* the top waiter priority (kernel view),
* @task lost.
*/
if (task->prio >= rt_mutex_top_waiter(lock)->prio)
return 0;
/*
* The current top waiter stays enqueued. We
* don't have to change anything in the lock
* waiters order.
*/
} else {
/*
* No waiters. Take the lock without the
* pi_lock dance.@task->pi_blocked_on is NULL
* and we have no waiters to enqueue in @task
* pi waiters list.
*/
goto takeit;
}
}
/*
* Clear @task->pi_blocked_on. Requires protection by
* @task->pi_lock. Redundant operation for the @waiter == NULL
* case, but conditionals are more expensive than a redundant
* store.
*/
raw_spin_lock_irqsave(&task->pi_lock, flags);
task->pi_blocked_on = NULL;
/*
* Finish the lock acquisition. @task is the new owner. If
* other waiters exist we have to insert the highest priority
* waiter into @task->pi_waiters list.
*/
if (rt_mutex_has_waiters(lock))
rt_mutex_enqueue_pi(task, rt_mutex_top_waiter(lock));
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
takeit:
/* We got the lock. */
debug_rt_mutex_lock(lock);
/*
* This either preserves the RT_MUTEX_HAS_WAITERS bit if there
* are still waiters or clears it.
*/
rt_mutex_set_owner(lock, task);
rt_mutex_deadlock_account_lock(lock, task);
return 1;
}
/*
* Task blocks on lock.
*
* Prepare waiter and propagate pi chain
*
* This must be called with lock->wait_lock held.
*/
static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
struct rt_mutex_waiter *waiter,
struct task_struct *task,
enum rtmutex_chainwalk chwalk)
{
struct task_struct *owner = rt_mutex_owner(lock);
struct rt_mutex_waiter *top_waiter = waiter;
struct rt_mutex *next_lock;
int chain_walk = 0, res;
unsigned long flags;
/*
* Early deadlock detection. We really don't want the task to
* enqueue on itself just to untangle the mess later. It's not
* only an optimization. We drop the locks, so another waiter
* can come in before the chain walk detects the deadlock. So
* the other will detect the deadlock and return -EDEADLOCK,
* which is wrong, as the other waiter is not in a deadlock
* situation.
*/
if (owner == task)
return -EDEADLK;
raw_spin_lock_irqsave(&task->pi_lock, flags);
__rt_mutex_adjust_prio(task);
waiter->task = task;
waiter->lock = lock;
waiter->prio = task->prio;
/* Get the top priority waiter on the lock */
if (rt_mutex_has_waiters(lock))
top_waiter = rt_mutex_top_waiter(lock);
rt_mutex_enqueue(lock, waiter);
task->pi_blocked_on = waiter;
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
if (!owner)
return 0;
raw_spin_lock_irqsave(&owner->pi_lock, flags);
if (waiter == rt_mutex_top_waiter(lock)) {
rt_mutex_dequeue_pi(owner, top_waiter);
rt_mutex_enqueue_pi(owner, waiter);
__rt_mutex_adjust_prio(owner);
if (owner->pi_blocked_on)
chain_walk = 1;
} else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) {
chain_walk = 1;
}
/* Store the lock on which owner is blocked or NULL */
next_lock = task_blocked_on_lock(owner);
raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
/*
* Even if full deadlock detection is on, if the owner is not
* blocked itself, we can avoid finding this out in the chain
* walk.
*/
if (!chain_walk || !next_lock)
return 0;
/*
* The owner can't disappear while holding a lock,
* so the owner struct is protected by wait_lock.
* Gets dropped in rt_mutex_adjust_prio_chain()!
*/
get_task_struct(owner);
raw_spin_unlock(&lock->wait_lock);
res = rt_mutex_adjust_prio_chain(owner, chwalk, lock,
next_lock, waiter, task);
raw_spin_lock(&lock->wait_lock);
return res;
}
/*
* Wake up the next waiter on the lock.
*
* Remove the top waiter from the current tasks pi waiter list and
* wake it up.
*
* Called with lock->wait_lock held.
*/
static void wakeup_next_waiter(struct rt_mutex *lock)
{
struct rt_mutex_waiter *waiter;
unsigned long flags;
raw_spin_lock_irqsave(¤t->pi_lock, flags);
waiter = rt_mutex_top_waiter(lock);
/*
* Remove it from current->pi_waiters. We do not adjust a
* possible priority boost right now. We execute wakeup in the
* boosted mode and go back to normal after releasing
* lock->wait_lock.
*/
rt_mutex_dequeue_pi(current, waiter);
/*
* As we are waking up the top waiter, and the waiter stays
* queued on the lock until it gets the lock, this lock
* obviously has waiters. Just set the bit here and this has
* the added benefit of forcing all new tasks into the
* slow path making sure no task of lower priority than
* the top waiter can steal this lock.
*/
lock->owner = (void *) RT_MUTEX_HAS_WAITERS;
raw_spin_unlock_irqrestore(¤t->pi_lock, flags);
/*
* It's safe to dereference waiter as it cannot go away as
* long as we hold lock->wait_lock. The waiter task needs to
* acquire it in order to dequeue the waiter.
*/
wake_up_process(waiter->task);
}
/*
* Remove a waiter from a lock and give up
*
* Must be called with lock->wait_lock held and
* have just failed to try_to_take_rt_mutex().
*/
static void remove_waiter(struct rt_mutex *lock,
struct rt_mutex_waiter *waiter)
{
bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
struct task_struct *owner = rt_mutex_owner(lock);
struct rt_mutex *next_lock;
unsigned long flags;
raw_spin_lock_irqsave(¤t->pi_lock, flags);
rt_mutex_dequeue(lock, waiter);
current->pi_blocked_on = NULL;
raw_spin_unlock_irqrestore(¤t->pi_lock, flags);
/*
* Only update priority if the waiter was the highest priority
* waiter of the lock and there is an owner to update.
*/
if (!owner || !is_top_waiter)
return;
raw_spin_lock_irqsave(&owner->pi_lock, flags);
rt_mutex_dequeue_pi(owner, waiter);
if (rt_mutex_has_waiters(lock))
rt_mutex_enqueue_pi(owner, rt_mutex_top_waiter(lock));
__rt_mutex_adjust_prio(owner);
/* Store the lock on which owner is blocked or NULL */
next_lock = task_blocked_on_lock(owner);
raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
/*
* Don't walk the chain, if the owner task is not blocked
* itself.
*/
if (!next_lock)
return;
/* gets dropped in rt_mutex_adjust_prio_chain()! */
get_task_struct(owner);
raw_spin_unlock(&lock->wait_lock);
rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock,
next_lock, NULL, current);
raw_spin_lock(&lock->wait_lock);
}
/*
* Recheck the pi chain, in case we got a priority setting
*
* Called from sched_setscheduler
*/
void rt_mutex_adjust_pi(struct task_struct *task)
{
struct rt_mutex_waiter *waiter;
struct rt_mutex *next_lock;
unsigned long flags;
raw_spin_lock_irqsave(&task->pi_lock, flags);
waiter = task->pi_blocked_on;
if (!waiter || (waiter->prio == task->prio &&
!dl_prio(task->prio))) {
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
return;
}
next_lock = waiter->lock;
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
/* gets dropped in rt_mutex_adjust_prio_chain()! */
get_task_struct(task);
rt_mutex_adjust_prio_chain(task, RT_MUTEX_MIN_CHAINWALK, NULL,
next_lock, NULL, task);
}
/**
* __rt_mutex_slowlock() - Perform the wait-wake-try-to-take loop
* @lock: the rt_mutex to take
* @state: the state the task should block in (TASK_INTERRUPTIBLE
* or TASK_UNINTERRUPTIBLE)
* @timeout: the pre-initialized and started timer, or NULL for none
* @waiter: the pre-initialized rt_mutex_waiter
*
* lock->wait_lock must be held by the caller.
*/
static int __sched
__rt_mutex_slowlock(struct rt_mutex *lock, int state,
struct hrtimer_sleeper *timeout,
struct rt_mutex_waiter *waiter)
{
int ret = 0;
for (;;) {
/* Try to acquire the lock: */
if (try_to_take_rt_mutex(lock, current, waiter))
break;
/*
* TASK_INTERRUPTIBLE checks for signals and
* timeout. Ignored otherwise.
*/
if (unlikely(state == TASK_INTERRUPTIBLE)) {
/* Signal pending? */
if (signal_pending(current))
ret = -EINTR;
if (timeout && !timeout->task)
ret = -ETIMEDOUT;
if (ret)
break;
}
raw_spin_unlock(&lock->wait_lock);
debug_rt_mutex_print_deadlock(waiter);
schedule_rt_mutex(lock);
raw_spin_lock(&lock->wait_lock);
set_current_state(state);
}
return ret;
}
static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
struct rt_mutex_waiter *w)
{
/*
* If the result is not -EDEADLOCK or the caller requested
* deadlock detection, nothing to do here.
*/
if (res != -EDEADLOCK || detect_deadlock)
return;
/*
* Yell lowdly and stop the task right here.
*/
rt_mutex_print_deadlock(w);
while (1) {
set_current_state(TASK_INTERRUPTIBLE);
schedule();
}
}
/*
* Slow path lock function:
*/
static int __sched
rt_mutex_slowlock(struct rt_mutex *lock, int state,
struct hrtimer_sleeper *timeout,
enum rtmutex_chainwalk chwalk)
{
struct rt_mutex_waiter waiter;
int ret = 0;
debug_rt_mutex_init_waiter(&waiter);
RB_CLEAR_NODE(&waiter.pi_tree_entry);
RB_CLEAR_NODE(&waiter.tree_entry);
raw_spin_lock(&lock->wait_lock);
/* Try to acquire the lock again: */
if (try_to_take_rt_mutex(lock, current, NULL)) {
raw_spin_unlock(&lock->wait_lock);
return 0;
}
set_current_state(state);
/* Setup the timer, when timeout != NULL */
if (unlikely(timeout)) {
hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
if (!hrtimer_active(&timeout->timer))
timeout->task = NULL;
}
ret = task_blocks_on_rt_mutex(lock, &waiter, current, chwalk);
if (likely(!ret))
ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);
set_current_state(TASK_RUNNING);
if (unlikely(ret)) {
remove_waiter(lock, &waiter);
rt_mutex_handle_deadlock(ret, chwalk, &waiter);
}
/*
* try_to_take_rt_mutex() sets the waiter bit
* unconditionally. We might have to fix that up.
*/
fixup_rt_mutex_waiters(lock);
raw_spin_unlock(&lock->wait_lock);
/* Remove pending timer: */
if (unlikely(timeout))
hrtimer_cancel(&timeout->timer);
debug_rt_mutex_free_waiter(&waiter);
return ret;
}
/*
* Slow path try-lock function:
*/
static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
{
int ret;
/*
* If the lock already has an owner we fail to get the lock.
* This can be done without taking the @lock->wait_lock as
* it is only being read, and this is a trylock anyway.
*/
if (rt_mutex_owner(lock))
return 0;
/*
* The mutex has currently no owner. Lock the wait lock and
* try to acquire the lock.
*/
raw_spin_lock(&lock->wait_lock);
ret = try_to_take_rt_mutex(lock, current, NULL);
/*
* try_to_take_rt_mutex() sets the lock waiters bit
* unconditionally. Clean this up.
*/
fixup_rt_mutex_waiters(lock);
raw_spin_unlock(&lock->wait_lock);
return ret;
}
/*
* Slow path to release a rt-mutex:
*/
static void __sched
rt_mutex_slowunlock(struct rt_mutex *lock)
{
raw_spin_lock(&lock->wait_lock);
debug_rt_mutex_unlock(lock);
rt_mutex_deadlock_account_unlock(current);
/*
* We must be careful here if the fast path is enabled. If we
* have no waiters queued we cannot set owner to NULL here
* because of:
*
* foo->lock->owner = NULL;
* rtmutex_lock(foo->lock); <- fast path
* free = atomic_dec_and_test(foo->refcnt);
* rtmutex_unlock(foo->lock); <- fast path
* if (free)
* kfree(foo);
* raw_spin_unlock(foo->lock->wait_lock);
*
* So for the fastpath enabled kernel:
*
* Nothing can set the waiters bit as long as we hold
* lock->wait_lock. So we do the following sequence:
*
* owner = rt_mutex_owner(lock);
* clear_rt_mutex_waiters(lock);
* raw_spin_unlock(&lock->wait_lock);
* if (cmpxchg(&lock->owner, owner, 0) == owner)
* return;
* goto retry;
*
* The fastpath disabled variant is simple as all access to
* lock->owner is serialized by lock->wait_lock:
*
* lock->owner = NULL;
* raw_spin_unlock(&lock->wait_lock);
*/
while (!rt_mutex_has_waiters(lock)) {
/* Drops lock->wait_lock ! */
if (unlock_rt_mutex_safe(lock) == true)
return;
/* Relock the rtmutex and try again */
raw_spin_lock(&lock->wait_lock);
}
/*
* The wakeup next waiter path does not suffer from the above
* race. See the comments there.
*/
wakeup_next_waiter(lock);
raw_spin_unlock(&lock->wait_lock);
/* Undo pi boosting if necessary: */
rt_mutex_adjust_prio(current);
}
/*
* debug aware fast / slowpath lock,trylock,unlock
*
* The atomic acquire/release ops are compiled away, when either the
* architecture does not support cmpxchg or when debugging is enabled.
*/
static inline int
rt_mutex_fastlock(struct rt_mutex *lock, int state,
int (*slowfn)(struct rt_mutex *lock, int state,
struct hrtimer_sleeper *timeout,
enum rtmutex_chainwalk chwalk))
{
if (likely(rt_mutex_cmpxchg(lock, NULL, current))) {
rt_mutex_deadlock_account_lock(lock, current);
return 0;
} else
return slowfn(lock, state, NULL, RT_MUTEX_MIN_CHAINWALK);
}
static inline int
rt_mutex_timed_fastlock(struct rt_mutex *lock, int state,
struct hrtimer_sleeper *timeout,
enum rtmutex_chainwalk chwalk,
int (*slowfn)(struct rt_mutex *lock, int state,
struct hrtimer_sleeper *timeout,
enum rtmutex_chainwalk chwalk))
{
if (chwalk == RT_MUTEX_MIN_CHAINWALK &&
likely(rt_mutex_cmpxchg(lock, NULL, current))) {
rt_mutex_deadlock_account_lock(lock, current);
return 0;
} else
return slowfn(lock, state, timeout, chwalk);
}
static inline int
rt_mutex_fasttrylock(struct rt_mutex *lock,
int (*slowfn)(struct rt_mutex *lock))
{
if (likely(rt_mutex_cmpxchg(lock, NULL, current))) {
rt_mutex_deadlock_account_lock(lock, current);
return 1;
}
return slowfn(lock);
}
static inline void
rt_mutex_fastunlock(struct rt_mutex *lock,
void (*slowfn)(struct rt_mutex *lock))
{
if (likely(rt_mutex_cmpxchg(lock, current, NULL)))
rt_mutex_deadlock_account_unlock(current);
else
slowfn(lock);
}
/**
* rt_mutex_lock - lock a rt_mutex
*
* @lock: the rt_mutex to be locked
*/
void __sched rt_mutex_lock(struct rt_mutex *lock)
{
might_sleep();
rt_mutex_fastlock(lock, TASK_UNINTERRUPTIBLE, rt_mutex_slowlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_lock);
/**
* rt_mutex_lock_interruptible - lock a rt_mutex interruptible
*
* @lock: the rt_mutex to be locked
*
* Returns:
* 0 on success
* -EINTR when interrupted by a signal
*/
int __sched rt_mutex_lock_interruptible(struct rt_mutex *lock)
{
might_sleep();
return rt_mutex_fastlock(lock, TASK_INTERRUPTIBLE, rt_mutex_slowlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_lock_interruptible);
/*
* Futex variant with full deadlock detection.
*/
int rt_mutex_timed_futex_lock(struct rt_mutex *lock,
struct hrtimer_sleeper *timeout)
{
might_sleep();
return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout,
RT_MUTEX_FULL_CHAINWALK,
rt_mutex_slowlock);
}
/**
* rt_mutex_timed_lock - lock a rt_mutex interruptible
* the timeout structure is provided
* by the caller
*
* @lock: the rt_mutex to be locked
* @timeout: timeout structure or NULL (no timeout)
*
* Returns:
* 0 on success
* -EINTR when interrupted by a signal
* -ETIMEDOUT when the timeout expired
*/
int
rt_mutex_timed_lock(struct rt_mutex *lock, struct hrtimer_sleeper *timeout)
{
might_sleep();
return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout,
RT_MUTEX_MIN_CHAINWALK,
rt_mutex_slowlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_timed_lock);
/**
* rt_mutex_trylock - try to lock a rt_mutex
*
* @lock: the rt_mutex to be locked
*
* Returns 1 on success and 0 on contention
*/
int __sched rt_mutex_trylock(struct rt_mutex *lock)
{
return rt_mutex_fasttrylock(lock, rt_mutex_slowtrylock);
}
EXPORT_SYMBOL_GPL(rt_mutex_trylock);
/**
* rt_mutex_unlock - unlock a rt_mutex
*
* @lock: the rt_mutex to be unlocked
*/
void __sched rt_mutex_unlock(struct rt_mutex *lock)
{
rt_mutex_fastunlock(lock, rt_mutex_slowunlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_unlock);
/**
* rt_mutex_destroy - mark a mutex unusable
* @lock: the mutex to be destroyed
*
* This function marks the mutex uninitialized, and any subsequent
* use of the mutex is forbidden. The mutex must not be locked when
* this function is called.
*/
void rt_mutex_destroy(struct rt_mutex *lock)
{
WARN_ON(rt_mutex_is_locked(lock));
#ifdef CONFIG_DEBUG_RT_MUTEXES
lock->magic = NULL;
#endif
}
EXPORT_SYMBOL_GPL(rt_mutex_destroy);
/**
* __rt_mutex_init - initialize the rt lock
*
* @lock: the rt lock to be initialized
*
* Initialize the rt lock to unlocked state.
*
* Initializing of a locked rt lock is not allowed
*/
void __rt_mutex_init(struct rt_mutex *lock, const char *name)
{
lock->owner = NULL;
raw_spin_lock_init(&lock->wait_lock);
lock->waiters = RB_ROOT;
lock->waiters_leftmost = NULL;
debug_rt_mutex_init(lock, name);
}
EXPORT_SYMBOL_GPL(__rt_mutex_init);
/**
* rt_mutex_init_proxy_locked - initialize and lock a rt_mutex on behalf of a
* proxy owner
*
* @lock: the rt_mutex to be locked
* @proxy_owner:the task to set as owner
*
* No locking. Caller has to do serializing itself
* Special API call for PI-futex support
*/
void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
struct task_struct *proxy_owner)
{
__rt_mutex_init(lock, NULL);
debug_rt_mutex_proxy_lock(lock, proxy_owner);
rt_mutex_set_owner(lock, proxy_owner);
rt_mutex_deadlock_account_lock(lock, proxy_owner);
}
/**
* rt_mutex_proxy_unlock - release a lock on behalf of owner
*
* @lock: the rt_mutex to be locked
*
* No locking. Caller has to do serializing itself
* Special API call for PI-futex support
*/
void rt_mutex_proxy_unlock(struct rt_mutex *lock,
struct task_struct *proxy_owner)
{
debug_rt_mutex_proxy_unlock(lock);
rt_mutex_set_owner(lock, NULL);
rt_mutex_deadlock_account_unlock(proxy_owner);
}
/**
* rt_mutex_start_proxy_lock() - Start lock acquisition for another task
* @lock: the rt_mutex to take
* @waiter: the pre-initialized rt_mutex_waiter
* @task: the task to prepare
*
* Returns:
* 0 - task blocked on lock
* 1 - acquired the lock for task, caller should wake it up
* <0 - error
*
* Special API call for FUTEX_REQUEUE_PI support.
*/
int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
struct rt_mutex_waiter *waiter,
struct task_struct *task)
{
int ret;
raw_spin_lock(&lock->wait_lock);
if (try_to_take_rt_mutex(lock, task, NULL)) {
raw_spin_unlock(&lock->wait_lock);
return 1;
}
/* We enforce deadlock detection for futexes */
ret = task_blocks_on_rt_mutex(lock, waiter, task,
RT_MUTEX_FULL_CHAINWALK);
if (ret && !rt_mutex_owner(lock)) {
/*
* Reset the return value. We might have
* returned with -EDEADLK and the owner
* released the lock while we were walking the
* pi chain. Let the waiter sort it out.
*/
ret = 0;
}
if (unlikely(ret))
remove_waiter(lock, waiter);
raw_spin_unlock(&lock->wait_lock);
debug_rt_mutex_print_deadlock(waiter);
return ret;
}
/**
* rt_mutex_next_owner - return the next owner of the lock
*
* @lock: the rt lock query
*
* Returns the next owner of the lock or NULL
*
* Caller has to serialize against other accessors to the lock
* itself.
*
* Special API call for PI-futex support
*/
struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)
{
if (!rt_mutex_has_waiters(lock))
return NULL;
return rt_mutex_top_waiter(lock)->task;
}
/**
* rt_mutex_finish_proxy_lock() - Complete lock acquisition
* @lock: the rt_mutex we were woken on
* @to: the timeout, null if none. hrtimer should already have
* been started.
* @waiter: the pre-initialized rt_mutex_waiter
*
* Complete the lock acquisition started our behalf by another thread.
*
* Returns:
* 0 - success
* <0 - error, one of -EINTR, -ETIMEDOUT
*
* Special API call for PI-futex requeue support
*/
int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
struct hrtimer_sleeper *to,
struct rt_mutex_waiter *waiter)
{
int ret;
raw_spin_lock(&lock->wait_lock);
set_current_state(TASK_INTERRUPTIBLE);
ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);
set_current_state(TASK_RUNNING);
if (unlikely(ret))
remove_waiter(lock, waiter);
/*
* try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
* have to fix that up.
*/
fixup_rt_mutex_waiters(lock);
raw_spin_unlock(&lock->wait_lock);
return ret;
}