Messages in this thread Patch in this message | | | From | Wander Lairson Costa <> | Subject | [PATCH] rtmutex: ensure we wake up the top waiter | Date | Tue, 17 Jan 2023 14:26:49 -0300 |
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In task_blocked_on_lock() we save the owner, release the wait_lock and call rt_mutex_adjust_prio_chain(). Before we acquire the wait_lock again, the owner may release the lock and deboost.
rt_mutex_adjust_prio_chain() acquires the wait_lock. In the requeue phase, waiter may be initially in the top of the queue, but after dequeued and requeued it may no longer be true.
This scenario ends up waking the wrong task, which will verify it is no the top waiter and comes back to sleep. Now we have a situation in which no task is holding the lock but no one acquires it.
We can reproduce the bug in PREEMPT_RT with stress-ng:
while true; do stress-ng --sched deadline --sched-period 1000000000 \ --sched-runtime 800000000 --sched-deadline \ 1000000000 --mmapfork 23 -t 20 done
Signed-off-by: Wander Lairson Costa <wander@redhat.com> --- kernel/locking/rtmutex.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 010cf4e6d0b8..728f434de2bb 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -901,8 +901,9 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, * 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_state(waiter->task, waiter->wake_state); + 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; } -- 2.39.0
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