// SPDX-License-Identifier: GPL-2.0-only

/*

* 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

* Adaptive Spinlocks:

* Copyright (C) 2008 Novell, Inc., Gregory Haskins, Sven Dietrich,

* and Peter Morreale,

* Adaptive Spinlocks simplification:

* Copyright (C) 2008 Red Hat, Inc., Steven Rostedt <srostedt@redhat.com>

*

* See Documentation/locking/rt-mutex-design.rst for details.

*/

#include <linux/sched.h>

#include <linux/sched/debug.h>

#include <linux/sched/deadline.h>

#include <linux/sched/signal.h>

#include <linux/sched/rt.h>

#include <linux/sched/wake_q.h>

#include <linux/ww_mutex.h>

#include <trace/events/lock.h>

#include "rtmutex_common.h"

#ifndef WW_RT

# define build_ww_mutex() (false)

# define ww_container_of(rtm) NULL

static inline int __ww_mutex_add_waiter(struct rt_mutex_waiter *waiter,

struct rt_mutex *lock,

struct ww_acquire_ctx *ww_ctx)

{

return 0;

}

static inline void __ww_mutex_check_waiters(struct rt_mutex *lock,

struct ww_acquire_ctx *ww_ctx)

{

}

static inline void ww_mutex_lock_acquired(struct ww_mutex *lock,

struct ww_acquire_ctx *ww_ctx)

{

}

static inline int __ww_mutex_check_kill(struct rt_mutex *lock,

struct rt_mutex_waiter *waiter,

struct ww_acquire_ctx *ww_ctx)

{

return 0;

}

#else

# define build_ww_mutex() (true)

# define ww_container_of(rtm) container_of(rtm, struct ww_mutex, base)

# include "ww_mutex.h"

#endif

/*

* 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 __always_inline struct task_struct *

rt_mutex_owner_encode(struct rt_mutex_base *lock, struct task_struct *owner)

{

unsigned long val = (unsigned long)owner;

if (rt_mutex_has_waiters(lock))

val |= RT_MUTEX_HAS_WAITERS;

return (struct task_struct *)val;

}

static __always_inline void

rt_mutex_set_owner(struct rt_mutex_base *lock, struct task_struct *owner)

{

/*

* lock->wait_lock is held but explicit acquire semantics are needed

* for a new lock owner so WRITE_ONCE is insufficient.

*/

xchg_acquire(&lock->owner, rt_mutex_owner_encode(lock, owner));

}

static __always_inline void rt_mutex_clear_owner(struct rt_mutex_base *lock)

{

/* lock->wait_lock is held so the unlock provides release semantics. */

WRITE_ONCE(lock->owner, rt_mutex_owner_encode(lock, NULL));

}

static __always_inline void clear_rt_mutex_waiters(struct rt_mutex_base *lock)

{

lock->owner = (struct task_struct *)

((unsigned long)lock->owner & ~RT_MUTEX_HAS_WAITERS);

}

static __always_inline void

fixup_rt_mutex_waiters(struct rt_mutex_base *lock, bool acquire_lock)

{

unsigned long owner, *p = (unsigned long *) &lock->owner;

if (rt_mutex_has_waiters(lock))

return;

/*

* The rbtree has no waiters enqueued, now make sure that the

* lock->owner still has the waiters bit set, otherwise the

* following can happen:

*

* CPU 0 CPU 1 CPU2

* l->owner=T1

* rt_mutex_lock(l)

* lock(l->lock)

* l->owner = T1 | HAS_WAITERS;

* enqueue(T2)

* boost()

* unlock(l->lock)

* block()

*

* rt_mutex_lock(l)

* lock(l->lock)

* l->owner = T1 | HAS_WAITERS;

* enqueue(T3)

* boost()

* unlock(l->lock)

* block()

* signal(->T2) signal(->T3)

* lock(l->lock)

* dequeue(T2)

* deboost()

* unlock(l->lock)

* lock(l->lock)

* dequeue(T3)

* ==> wait list is empty

* deboost()

* unlock(l->lock)

* lock(l->lock)

* fixup_rt_mutex_waiters()

* if (wait_list_empty(l) {

* l->owner = owner

* owner = l->owner & ~HAS_WAITERS;

* ==> l->owner = T1

* }

* lock(l->lock)

* rt_mutex_unlock(l) fixup_rt_mutex_waiters()

* if (wait_list_empty(l) {

* owner = l->owner & ~HAS_WAITERS;

* cmpxchg(l->owner, T1, NULL)

* ===> Success (l->owner = NULL)

*

* l->owner = owner

* ==> l->owner = T1

* }

*

* With the check for the waiter bit in place T3 on CPU2 will not

* overwrite. All tasks fiddling with the waiters bit are

* serialized by l->lock, so nothing else can modify the waiters

* bit. If the bit is set then nothing can change l->owner either

* so the simple RMW is safe. The cmpxchg() will simply fail if it

* happens in the middle of the RMW because the waiters bit is

* still set.

*/

owner = READ_ONCE(*p);

if (owner & RT_MUTEX_HAS_WAITERS) {

/*

* See rt_mutex_set_owner() and rt_mutex_clear_owner() on

* why xchg_acquire() is used for updating owner for

* locking and WRITE_ONCE() for unlocking.

*

* WRITE_ONCE() would work for the acquire case too, but

* in case that the lock acquisition failed it might

* force other lockers into the slow path unnecessarily.

*/

if (acquire_lock)

xchg_acquire(p, owner & ~RT_MUTEX_HAS_WAITERS);

else

WRITE_ONCE(*p, owner & ~RT_MUTEX_HAS_WAITERS);

}

}

/*

* We can speed up the acquire/release, if there's no debugging state to be

* set up.

*/

#ifndef CONFIG_DEBUG_RT_MUTEXES

static __always_inline bool rt_mutex_cmpxchg_acquire(struct rt_mutex_base *lock,

struct task_struct *old,

struct task_struct *new)

{

return try_cmpxchg_acquire(&lock->owner, &old, new);

}

static __always_inline bool rt_mutex_try_acquire(struct rt_mutex_base *lock)

{

return rt_mutex_cmpxchg_acquire(lock, NULL, current);

}

static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,

struct task_struct *old,

struct task_struct *new)

{

return try_cmpxchg_release(&lock->owner, &old, new);

}

/*

* Callers must hold the ->wait_lock -- which is the whole purpose as we force

* all future threads that attempt to [Rmw] the lock to the slowpath. As such

* relaxed semantics suffice.

*/

static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)

{

unsigned long *p = (unsigned long *) &lock->owner;

unsigned long owner, new;

owner = READ_ONCE(*p);

do {

new = owner | RT_MUTEX_HAS_WAITERS;

} while (!try_cmpxchg_relaxed(p, &owner, new));

/*

* The cmpxchg loop above is relaxed to avoid back-to-back ACQUIRE

* operations in the event of contention. Ensure the successful

* cmpxchg is visible.

*/

smp_mb__after_atomic();

}

/*

* Safe fastpath aware unlock:

* 1) Clear the waiters bit

* 2) Drop lock->wait_lock

* 3) Try to unlock the lock with cmpxchg

*/

static __always_inline bool unlock_rt_mutex_safe(struct rt_mutex_base *lock,

unsigned long flags)

__releases(lock->wait_lock)

{

struct task_struct *owner = rt_mutex_owner(lock);

clear_rt_mutex_waiters(lock);

raw_spin_unlock_irqrestore(&lock->wait_lock, flags);

/*

* 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_release(lock, owner, NULL);

}

#else

static __always_inline bool rt_mutex_cmpxchg_acquire(struct rt_mutex_base *lock,

struct task_struct *old,

struct task_struct *new)

{

return false;

}

static int __sched rt_mutex_slowtrylock(struct rt_mutex_base *lock);

static __always_inline bool rt_mutex_try_acquire(struct rt_mutex_base *lock)

{

/*

* With debug enabled rt_mutex_cmpxchg trylock() will always fail.

*

* Avoid unconditionally taking the slow path by using

* rt_mutex_slow_trylock() which is covered by the debug code and can

* acquire a non-contended rtmutex.

*/

return rt_mutex_slowtrylock(lock);

}

static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,

struct task_struct *old,

struct task_struct *new)

{

return false;

}

static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *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 __always_inline bool unlock_rt_mutex_safe(struct rt_mutex_base *lock,

unsigned long flags)

__releases(lock->wait_lock)

{

lock->owner = NULL;

raw_spin_unlock_irqrestore(&lock->wait_lock, flags);

return true;

}

#endif

static __always_inline int __waiter_prio(struct task_struct *task)

{

int prio = task->prio;

if (!rt_prio(prio))

return DEFAULT_PRIO;

return prio;

}

/*

* Update the waiter->tree copy of the sort keys.

*/

static __always_inline void

waiter_update_prio(struct rt_mutex_waiter *waiter, struct task_struct *task)

{

lockdep_assert_held(&waiter->lock->wait_lock);

lockdep_assert(RB_EMPTY_NODE(&waiter->tree.entry));

waiter->tree.prio = __waiter_prio(task);

waiter->tree.deadline = task->dl.deadline;

}

/*

* Update the waiter->pi_tree copy of the sort keys (from the tree copy).

*/

static __always_inline void

waiter_clone_prio(struct rt_mutex_waiter *waiter, struct task_struct *task)

{

lockdep_assert_held(&waiter->lock->wait_lock);

lockdep_assert_held(&task->pi_lock);

lockdep_assert(RB_EMPTY_NODE(&waiter->pi_tree.entry));

waiter->pi_tree.prio = waiter->tree.prio;

waiter->pi_tree.deadline = waiter->tree.deadline;

}

/*

* Only use with rt_waiter_node_{less,equal}()

*/

#define task_to_waiter_node(p) \

&(struct rt_waiter_node){ .prio = __waiter_prio(p), .deadline = (p)->dl.deadline }

#define task_to_waiter(p) \

&(struct rt_mutex_waiter){ .tree = *task_to_waiter_node(p) }

static __always_inline int rt_waiter_node_less(struct rt_waiter_node *left,

struct rt_waiter_node *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 dl_time_before(left->deadline, right->deadline);

return 0;

}

static __always_inline int rt_waiter_node_equal(struct rt_waiter_node *left,

struct rt_waiter_node *right)

{

if (left->prio != right->prio)

return 0;

/*

* 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 0 above,

* then right waiter has a dl_prio() too.

*/

if (dl_prio(left->prio))

return left->deadline == right->deadline;

return 1;

}

static inline bool rt_mutex_steal(struct rt_mutex_waiter *waiter,

struct rt_mutex_waiter *top_waiter)

{

if (rt_waiter_node_less(&waiter->tree, &top_waiter->tree))

return true;

#ifdef RT_MUTEX_BUILD_SPINLOCKS

/*

* Note that RT tasks are excluded from same priority (lateral)

* steals to prevent the introduction of an unbounded latency.

*/

if (rt_prio(waiter->tree.prio) || dl_prio(waiter->tree.prio))

return false;

return rt_waiter_node_equal(&waiter->tree, &top_waiter->tree);

#else

return false;

#endif

}

#define __node_2_waiter(node) \

rb_entry((node), struct rt_mutex_waiter, tree.entry)

static __always_inline bool __waiter_less(struct rb_node *a, const struct rb_node *b)

{

struct rt_mutex_waiter *aw = __node_2_waiter(a);

struct rt_mutex_waiter *bw = __node_2_waiter(b);

if (rt_waiter_node_less(&aw->tree, &bw->tree))

return 1;

if (!build_ww_mutex())

return 0;

if (rt_waiter_node_less(&bw->tree, &aw->tree))

return 0;

/* NOTE: relies on waiter->ww_ctx being set before insertion */

if (aw->ww_ctx) {

if (!bw->ww_ctx)

return 1;

return (signed long)(aw->ww_ctx->stamp -

bw->ww_ctx->stamp) < 0;

}

return 0;

}

static __always_inline void

rt_mutex_enqueue(struct rt_mutex_base *lock, struct rt_mutex_waiter *waiter)

{

lockdep_assert_held(&lock->wait_lock);

rb_add_cached(&waiter->tree.entry, &lock->waiters, __waiter_less);

}

static __always_inline void

rt_mutex_dequeue(struct rt_mutex_base *lock, struct rt_mutex_waiter *waiter)

{

lockdep_assert_held(&lock->wait_lock);

if (RB_EMPTY_NODE(&waiter->tree.entry))

return;

rb_erase_cached(&waiter->tree.entry, &lock->waiters);

RB_CLEAR_NODE(&waiter->tree.entry);

}

#define __node_2_rt_node(node) \

rb_entry((node), struct rt_waiter_node, entry)

static __always_inline bool __pi_waiter_less(struct rb_node *a, const struct rb_node *b)

{

return rt_waiter_node_less(__node_2_rt_node(a), __node_2_rt_node(b));

}

static __always_inline void

rt_mutex_enqueue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)

{

lockdep_assert_held(&task->pi_lock);

rb_add_cached(&waiter->pi_tree.entry, &task->pi_waiters, __pi_waiter_less);

}

static __always_inline void

rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)

{

lockdep_assert_held(&task->pi_lock);

if (RB_EMPTY_NODE(&waiter->pi_tree.entry))

return;

rb_erase_cached(&waiter->pi_tree.entry, &task->pi_waiters);

RB_CLEAR_NODE(&waiter->pi_tree.entry);

}

static __always_inline void rt_mutex_adjust_prio(struct rt_mutex_base *lock,

struct task_struct *p)

{

struct task_struct *pi_task = NULL;

lockdep_assert_held(&lock->wait_lock);

lockdep_assert(rt_mutex_owner(lock) == p);

lockdep_assert_held(&p->pi_lock);

if (task_has_pi_waiters(p))

pi_task = task_top_pi_waiter(p)->task;

rt_mutex_setprio(p, pi_task);

}

/* RT mutex specific wake_q wrappers */

static __always_inline void rt_mutex_wake_q_add_task(struct rt_wake_q_head *wqh,

struct task_struct *task,

unsigned int wake_state)

{

if (IS_ENABLED(CONFIG_PREEMPT_RT) && wake_state == TASK_RTLOCK_WAIT) {

if (IS_ENABLED(CONFIG_PROVE_LOCKING))

WARN_ON_ONCE(wqh->rtlock_task);

get_task_struct(task);

wqh->rtlock_task = task;

} else {

wake_q_add(&wqh->head, task);

}

}

static __always_inline void rt_mutex_wake_q_add(struct rt_wake_q_head *wqh,

struct rt_mutex_waiter *w)

{

rt_mutex_wake_q_add_task(wqh, w->task, w->wake_state);

}

static __always_inline void rt_mutex_wake_up_q(struct rt_wake_q_head *wqh)

{

if (IS_ENABLED(CONFIG_PREEMPT_RT) && wqh->rtlock_task) {

wake_up_state(wqh->rtlock_task, TASK_RTLOCK_WAIT);

put_task_struct(wqh->rtlock_task);

wqh->rtlock_task = NULL;

}

if (!wake_q_empty(&wqh->head))

wake_up_q(&wqh->head);

/* Pairs with preempt_disable() in mark_wakeup_next_waiter() */

preempt_enable();

}

/*

* 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 __always_inline bool

rt_mutex_cond_detect_deadlock(struct rt_mutex_waiter *waiter,

enum rtmutex_chainwalk chwalk)

{

if (IS_ENABLED(CONFIG_DEBUG_RT_MUTEXES))

return waiter != NULL;

return chwalk == RT_MUTEX_FULL_CHAINWALK;

}

static __always_inline struct rt_mutex_base *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

* @chwalk: 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

* [Pn] task->pi_lock held

* [L] rtmutex->wait_lock held

*

* Normal locking order:

*

* rtmutex->wait_lock

* task->pi_lock

*

* 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 [P1]

* [2] waiter = task->pi_blocked_on; [P1]

* [3] check_exit_conditions_1(); [P1]

* [4] lock = waiter->lock; [P1]

* [5] if (!try_lock(lock->wait_lock)) { [P1] try to acquire [L]

* unlock(task->pi_lock); release [P1]

* goto retry;

* }

* [6] check_exit_conditions_2(); [P1] + [L]

* [7] requeue_lock_waiter(lock, waiter); [P1] + [L]

* [8] unlock(task->pi_lock); release [P1]

* 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 [P2]

* [11] requeue_pi_waiter(tsk, waiters(lock));[P2] + [L]

* [12] check_exit_conditions_4(); [P2] + [L]

* [13] unlock(task->pi_lock); release [P2]

* unlock(lock->wait_lock); release [L]

* goto again;

*

* Where P1 is the blocking task and P2 is the lock owner; going up one step

* the owner becomes the next blocked task etc..

*

*

*/

static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,

enum rtmutex_chainwalk chwalk,

struct rt_mutex_base *orig_lock,

struct rt_mutex_base *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_base *lock;

bool detect_deadlock;

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_irq(&task->pi_lock);

/*

* [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;

/*

* There could be 'spurious' loops in the lock graph due to ww_mutex,

* consider:

*

* P1: A, ww_A, ww_B

* P2: ww_B, ww_A

* P3: A

*

* P3 should not return -EDEADLK because it gets trapped in the cycle

* created by P1 and P2 (which will resolve -- and runs into

* max_lock_depth above). Therefore disable detect_deadlock such that

* the below termination condition can trigger once all relevant tasks

* are boosted.

*

* Even when we start with ww_mutex we can disable deadlock detection,

* since we would supress a ww_mutex induced deadlock at [6] anyway.

* Supressing it here however is not sufficient since we might still

* hit [6] due to adjustment driven iteration.

*

* NOTE: if someone were to create a deadlock between 2 ww_classes we'd

* utterly fail to report it; lockdep should.

*/

if (IS_ENABLED(CONFIG_PREEMPT_RT) && waiter->ww_ctx && detect_deadlock)

detect_deadlock = false;

/*

* 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 (rt_waiter_node_equal(&waiter->tree, task_to_waiter_node(task))) {

if (!detect_deadlock)

goto out_unlock_pi;

else

requeue = false;

}

/*

* [4] Get the next lock; per holding task->pi_lock we can't unblock

* and guarantee @lock's existence.

*/

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.

*

* Per the above, holding task->pi_lock guarantees lock exists, so

* inverting this lock order is infeasible from a life-time

* perspective.

*/

if (!raw_spin_trylock(&lock->wait_lock)) {

raw_spin_unlock_irq(&task->pi_lock);

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) {

ret = -EDEADLK;

/*

* When the deadlock is due to ww_mutex; also see above. Don't

* report the deadlock and instead let the ww_mutex wound/die

* logic pick which of the contending threads gets -EDEADLK.

*

* NOTE: assumes the cycle only contains a single ww_class; any

* other configuration and we fail to report; also, see

* lockdep.

*/

if (IS_ENABLED(CONFIG_PREEMPT_RT) && orig_waiter && orig_waiter->ww_ctx)

ret = 0;

raw_spin_unlock(&lock->wait_lock);

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(&task->pi_lock);

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_irq(&lock->wait_lock);

return 0;

}

/* [10] Grab the next task, i.e. owner of @lock */

task = get_task_struct(rt_mutex_owner(lock));

raw_spin_lock(&task->pi_lock);

/*

* 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(&task->pi_lock);

raw_spin_unlock_irq(&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 tree. */

rt_mutex_dequeue(lock, waiter);

/*

* Update the waiter prio fields now that we're dequeued.

*

* These values can have changed through either:

*

* sys_sched_set_scheduler() / sys_sched_setattr()

*

* or

*

* DL CBS enforcement advancing the effective deadline.

*/

waiter_update_prio(waiter, task);

rt_mutex_enqueue(lock, waiter);

/*

* [8] Release the (blocking) task in preparation for

* taking the owner task in [10].

*

* Since we hold lock->waiter_lock, task cannot unblock, even if we

* release task->pi_lock.

*/

raw_spin_unlock(&task->pi_lock);

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.

*/

top_waiter = rt_mutex_top_waiter(lock);

if (prerequeue_top_waiter != top_waiter)

wake_up_state(top_waiter->task, top_waiter->wake_state);

raw_spin_unlock_irq(&lock->wait_lock);

return 0;

}

/*

* [10] Grab the next task, i.e. the owner of @lock

*

* Per holding lock->wait_lock and checking for !owner above, there

* must be an owner and it cannot go away.

*/

task = get_task_struct(rt_mutex_owner(lock));

raw_spin_lock(&task->pi_lock);

/* [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 tree with this waiter

* and adjust the priority of the owner.

*/

rt_mutex_dequeue_pi(task, prerequeue_top_waiter);

waiter_clone_prio(waiter, task);

rt_mutex_enqueue_pi(task, waiter);

rt_mutex_adjust_prio(lock, task);