From 912edebe8501a36c6bedcef03bd238ab90a7e060 Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Tue, 1 Sep 2026 15:54:51 +0200 Subject: futex: Provide rt_mutex_.*_schedule() equivalents for futex scheduling There is rt_mutex_{pre|post}_schedule() around rt_mutex_wait_proxy_lock() to ensure that sched_submit_work()/ sched_update_worker() is invoked before we schedule out and block on rt_mutex while waiting for it become available. The reason is that blocking on rt_mutex assigns a pi_waiter for the PI chain and sched_submit_work() will also assign a pi_waiter if it blocks on lock but a this point we already have a waiter assigned. We can't skip sched_submit_work() entirely because I/O relies on the fact that I/O queue is flushed while it blocks on a sleeping lock. Therefore sched_submit_work() is moved before we block on the lock. Sleeping lock in this context means mutex or rw_semaphore not spinlock_t on PREEMPT_RT. Because the mutex abstraction on PREEMPT_RT uses the same abstraction as the futex proxy lock, the futex code ended up using rt_mutex_{pre|post}_schedule(), too. Using it is/ was just to keep the task_struct::sched_rt_mutex assertion happy. Futex proxy lock is used only in the syscall context of a task. At this point it never got any I/O that needs to be flushed and it can't be a workqueue that needs to notify that it will be scheduled out. Therefore sched_submit_work() does nothing here. By mistake futex_wait_requeue_pi() -> rt_mutex_wait_proxy_lock() did not get the rt_mutex_{pre|post}_schedule() annotation. This was not noticed because in this callchain the lock is (usually) not contended and so rt_mutex_slowlock_block() does not schedule, triggering the assert. Adding rt_mutex_pre_schedule() here looks wrong (as noted by PeterZ) because at this point there is a pi_waiter recorded and invoking sched_submit_work() with a possible lock contention would be wrong. Add rt_mutex_futex_{pre|post}_schedule() which toggles the sched_rt_mutex assert and does not involve sched_submit_work(). Add asserts here to ensure that sched_submit_work() would do nothing. Use it only in futex proxy lock case which is rt_mutex_wait_proxy_lock(). Remove it from futex_lock_pi(). Fixes: d14f9e930b90 ("locking/rtmutex: Use rt_mutex specific scheduler helpers") Reported-by: Yao Kai Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260901135453.3121948-2-bigeasy@linutronix.de Closes: https://lore.kernel.org/all/20260717084922.4153317-2-yaokai34@huawei.com --- kernel/locking/rtmutex_api.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'kernel/locking/rtmutex_api.c') diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c index 5d48d64725b1..eb18b094473c 100644 --- a/kernel/locking/rtmutex_api.c +++ b/kernel/locking/rtmutex_api.c @@ -423,6 +423,7 @@ int __sched rt_mutex_wait_proxy_lock(struct rt_mutex_base *lock, { int ret; + rt_mutex_futex_pre_schedule(); raw_spin_lock_irq(&lock->wait_lock); /* sleep on the mutex */ set_current_state(TASK_INTERRUPTIBLE); @@ -433,6 +434,7 @@ int __sched rt_mutex_wait_proxy_lock(struct rt_mutex_base *lock, */ fixup_rt_mutex_waiters(lock, true); raw_spin_unlock_irq(&lock->wait_lock); + rt_mutex_futex_post_schedule(); return ret; } -- cgit v1.2.3