Re: [PATCH] kthread: Enhance kthread_stop to abort interruptible sleeps

From: Eric W. Biederman
Date: Tue Apr 24 2007 - 13:20:36 EST

Oleg Nesterov <oleg@xxxxxxxxxx> writes:

> On 04/24, Oleg Nesterov wrote:
>> Hm, mm_release() clears ->vfork_done before complete().

Duh. Yes somehow I had a blind spot there. I clearly
need to handle that case.

>> mm_release:
>> struct completion *vfork_done = tsk->vfork_done;
>> if (vfork_done) {
>> tsk->vfork_done = NULL;
>> complete(vfork_done);
>> }
>> kthread_stop:
>> set_tsk_thread_flag(tsk, TIF_KTHREAD_STOP);
>> signal_wake_up(tsk, 1);
>> // tsk exits, sets ->vfork_done == NULL
>> wait_for_completion(tsk->vfork_done);
> Since the task_struct should be pinned anyway, I think kthread_stop()
> should do:
> vfork_done = tsk->vfork_done;
> barrier();
> if (vfork_done)
> wait_for_completion(vfork_done);

That should work, and this may explain what is going on. I was trying
to figure out how this could happen but if the thread is running on
another cpu there is a race and it may exit quickly enough to cause us
problems before we get to kthread_stop.

I don't know if this is the problem but it certainly needs to be fixed.


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