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Should we consider the race condition as blow? From 4035f50c96e17cbe3febab768b64da5c000e5b76 Mon Sep 17 00:00:00 2001 From: Yanfei Xu Date: Mon, 5 Jul 2021 17:56:58 +0800 Subject: [PATCH] locking/mutex: fix the endless loop when racing against mutex.owner if a race condition happened on mutex.owner after we fetch its value, atomic_long_try_cmpxchg_acquire/release invoked on &mutex.owner will return false. Then we need to reassign the temporary variable which saves mutex.owner value if in loop, or it will lead an endless loop. Fixes: 9265e48a579d ("locking/mutex: Use try_cmpxchg()") Signed-off-by: Yanfei Xu --- kernel/locking/mutex.c | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 5e6a811ac733..ec6b6724c118 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -95,12 +95,12 @@ static inline unsigned long __owner_flags(unsigned long owner) static inline struct task_struct *__mutex_trylock_common(struct mutex *lock, bool handoff) { - unsigned long owner, curr = (unsigned long)current; + unsigned long flags, owner, task, curr = (unsigned long)current; - owner = atomic_long_read(&lock->owner); for (;;) { /* must loop, can race against a flag */ - unsigned long flags = __owner_flags(owner); - unsigned long task = owner & ~MUTEX_FLAGS; + owner = atomic_long_read(&lock->owner); + flags = __owner_flags(owner); + task = owner & ~MUTEX_FLAGS; if (task) { if (flags & MUTEX_FLAG_PICKUP) { @@ -231,10 +231,10 @@ __mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter) */ static void __mutex_handoff(struct mutex *lock, struct task_struct *task) { - unsigned long owner = atomic_long_read(&lock->owner); + unsigned long owner, new; for (;;) { - unsigned long new; + owner = atomic_long_read(&lock->owner); MUTEX_WARN_ON(__owner_task(owner) != current); MUTEX_WARN_ON(owner & MUTEX_FLAG_PICKUP); @@ -1227,8 +1227,9 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne * Except when HANDOFF, in that case we must not clear the owner field, * but instead set it to the top waiter. */ - owner = atomic_long_read(&lock->owner); for (;;) { + owner = atomic_long_read(&lock->owner); + MUTEX_WARN_ON(__owner_task(owner) != current); MUTEX_WARN_ON(owner & MUTEX_FLAG_PICKUP); -- 2.29.2