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Vineeth found came up with a test driver that could trip up workqueue stalls. After fixing one issue this test found, Vineeth reported the test was still failing. Greatly simplified, a task that tries to take a mutex already owned by another task that is sleeping, can hit a edge case in the mutex_lock_common() case. If the task fails to get the lock, calls into schedule, but gets a spurious wakeup, it will find that it is first waiter, and go into the mutex_optimistic_spin() logic. Though before calling mutex_optimistic_spin(), we clear task blocked_on state, since mutex_optimistic_spin() may call schedule() if need_resched() is set. After mutex_optimistic_spin() fails, we set blocked_on again, restart the main mutex loop, try to take the lock and call into schedule_preempt_disabled(). From there, with proxy-execution, we'll see the task is blocked_on, follow the chain, see the owner is sleeping and dequeue the waiting task from the runqueue. This all sounds fine and reasonable. But what I had missed is that in mutex_optimistic_spin(), not only do we call schedule() but we set TASK_RUNNABLE right before doing so. This is ok for that invocation of schedule(). But when we come back we re-set the blocked_on we had just cleared, but we do not re-set the task state to TASK_INTERRUPTIBLE/UNINTERRUPTIBLE. This means we have a task that is blocked_on & TASK_RUNNABLE, so when the proxy execution code dequeues the task, we are in trouble since future wakeups will be shortcut by the ttwu_state_match() check. Thus, to avoid this, after mutex_optimistic_spin(), set the task state back when we set blocked_on. Many many thanks again to Vineeth for his very useful testing driver that uncovered this long hidden bug, that I hadn't tripped in all my testing! Very impressed with the problems he's uncovered! Reported-by: Vineeth Pillai <vineethrp@google.com> Signed-off-by: John Stultz <jstultz@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: Vineeth Pillai <vineethrp@google.com> Link: https://patch.msgid.link/20260430215103.2978955-3-jstultz@google.com |
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| .. | ||
| irqflag-debug.c | ||
| lock_events_list.h | ||
| lock_events.c | ||
| lock_events.h | ||
| lockdep_internals.h | ||
| lockdep_proc.c | ||
| lockdep_states.h | ||
| lockdep.c | ||
| locktorture.c | ||
| Makefile | ||
| mcs_spinlock.h | ||
| mutex-debug.c | ||
| mutex.c | ||
| mutex.h | ||
| osq_lock.c | ||
| percpu-rwsem.c | ||
| qrwlock.c | ||
| qspinlock_paravirt.h | ||
| qspinlock_stat.h | ||
| qspinlock.c | ||
| qspinlock.h | ||
| rtmutex_api.c | ||
| rtmutex_common.h | ||
| rtmutex.c | ||
| rwbase_rt.c | ||
| rwsem.c | ||
| semaphore.c | ||
| spinlock_debug.c | ||
| spinlock_rt.c | ||
| spinlock.c | ||
| test-ww_mutex.c | ||
| ww_mutex.h | ||
| ww_rt_mutex.c | ||