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SubjectRe: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs
On 08/08, Bart Van Assche wrote:
>
> This is the sequence of which I think that it leads to the missed wakeup:
>
> Task 1 Task 2 Task 3 Task 4
>
> lock_page()
> ...
> lock_page_killable()
> __lock_page_killable()
> __wait_on_bit_lock()
> bit_wait_io()
> io_schedule()
> ...
> lock_page()
> __lock_page()
> __wait_on_bit_lock()
> bit_wait_io()
> io_schedule()
> ...
>
>
> (signal delivery to task 2)
> try_to_wake_up(task2, ..., ...)
> (try_to_wake_up() returns 1)
>
> unlock_page()
> wake_up_page()
> __wake_up_bit()
> __wake_up(wq, TASK_NORMAL, 1, &key)
> __wake_up_common(wq, mode=TASK_NORMAL, nr_exclusive=1, 0, key)
> wake_bit_function()
> autoremove_wake_function()
> default_wake_function()
> try_to_wake_up() <- skips task 2 because task 3 already changed
> the task state of task 2
> (autoremove_wake_function() does not do
> list_del_init(&wait->task_list))

Yes.

But since it skips task2, __wake_up_common() doesn't decrement nr_exclusive,
doesn't stop. It continues the list_for_each_entry_safe() loop, and finds the
sleeping task4, and wakes it up,

> bit_wait_io() returns -EINTR
> abort_exclusive_wait() is called by __wait_on_bit_lock()
>
>
> In the above sequence task 1 does not remove task 2 from the waitqueue
> because task 3 had already woken up task 2. The result is that when task 2
> calls abort_exclusive_wait() that task 2 is still on the waitqueue.

Yes, but this is fine,

> With the
> current implementation of abort_exclusive_wait() in the above scenario task
> 4 is not woken up although it should be woken up.

See above, it must be already woken by __wake_up_common().



So far _I think_ that the bug is somewhere else... Say, someone clears
PG_locked without wake_up(). Then SIGKILL sent to the task sleeping in
sys_read() "adds" the necessary wakeup...

Do you use external modules during the testing?

Oleg.

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