Messages in this thread Patch in this message | | | From | Yanfei Xu <> | Subject | [PATCH] locking/mutex: fix the MUTEX_FLAG_HANDOFF bit is cleared unexpected | Date | Mon, 28 Jun 2021 23:51:55 +0800 |
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When the mutex unlock path is excuted with WAITERS bit and without HANDOFF bit set, it will wake up the first task in wait_list. If there are some tasks not in wait_list are stealing lock, it is very likely successfully due to the task field of lock is NULL and flags field is non-NULL. Then the HANDOFF bit will be cleared. But if the HANDOFF bit was just set by the waked task in wait_list, this clearing is unexcepted.
__mutex_lock_common __mutex_lock_common __mutex_trylock schedule_preempt_disabled /*steal lock successfully*/ __mutex_set_flag(lock,MUTEX_FLAG_HANDOFF) __mutex_trylock_or_owner if (task==NULL) flags &= ~MUTEX_FLAG_HANDOFF atomic_long_cmpxchg_acquire __mutex_trylock //failed mutex_optimistic_spin //failed schedule_preempt_disabled //sleep without HANDOFF bit
So the HANDOFF bit should be set as late as possible, here we defer it util the task is going to be scheduled.
Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com> ---
Hi maintainers,
I am not very sure if I missed or misunderstanded something, please help to review. Many thanks!
kernel/locking/mutex.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 013e1b08a1bf..e57d920e96bf 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -1033,17 +1033,17 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, } spin_unlock(&lock->wait_lock); + + if (first) + __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); schedule_preempt_disabled(); /* * ww_mutex needs to always recheck its position since its waiter * list is not FIFO ordered. */ - if (ww_ctx || !first) { + if (ww_ctx || !first) first = __mutex_waiter_is_first(lock, &waiter); - if (first) - __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); - } set_current_state(state); /* -- 2.27.0
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