On 08/08/16 03:22, Peter Zijlstra wrote:
> That would be the exact scenario I drew a picture of, no? I'm still
> failing to see the hole there.
>
> Please draw a picture like that and illustrate the hole.
Hi Peter,
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))
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. With the
current implementation of abort_exclusive_wait() in the above scenario task
4 is not woken up although it should be woken up. Hence the patch that removes
the "else" keyword from abort_exclusive_wait().
Bart.
