Messages in this thread | | | Date | Tue, 17 Jan 2023 14:40:22 -0500 | Subject | Re: [PATCH] rtmutex: ensure we wake up the top waiter | From | Waiman Long <> |
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On 1/17/23 14:32, Waiman Long wrote: > On 1/17/23 12:26, Wander Lairson Costa wrote: >> 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. > Are you referring to task_blocks_on_rt_mutex(), not > task_blocked_on_lock()? >> >> 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; >> } > > I would say that if a rt_mutex has no owner but have waiters, we > should always wake up the top waiter whether or not it is the current > waiter. So what is the result of the stress-ng run above? Is it a > hang? It is not clear from your patch description.
BTW, if it is a hang. What arch has this problem? x86 or arm64? There is a recent report of some rt_mutex locking issue in arm64, I believe. I don't know if it will be related. So it is important to know in what arch you see this problem.
Cheers, Longman
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