On Wed, Jun 20, 2012 at 09:53:31PM +0530, Aaditya Kumar wrote:
> Offlining memory may block forever, waiting for kswapd() to wake up because
> kswapd() does not check the event kthread->should_stop before sleeping.
>
> The proper pattern, from Documentation/memory-barriers.txt, is:
> --- waker ---
> event_indicated = 1;
> wake_up_process(event_daemon);
>
> --- sleeper ---
> for (;;) {
> set_current_state(TASK_UNINTERRUPTIBLE);
> if (event_indicated)
> break;
> schedule();
> }
>
> set_current_state() may be wrapped by:
> prepare_to_wait();
>
> In the kswapd() case, event_indicated is kthread->should_stop.
> --- offlining memory (waker) ---
> kswapd_stop()
> kthread_stop()
> kthread->should_stop = 1
> wake_up_process()
> wait_for_completion()
>
>
> --- kswapd_try_to_sleep (sleeper) ---
> kswapd_try_to_sleep()
> prepare_to_wait()
> .
> .
> schedule()
> .
> .
> finish_wait()
>
> The schedule() needs to be protected by a test of kthread->should_stop,
> which is wrapped by kthread_should_stop().
>
> Reproducer:
> Do heavy file I/O in background.
> Do a memory offline/online in a tight loop
>
>
> Signed-off-by: Aaditya Kumar <aaditya.kumar@xxxxxxxxxxx>
<formletter>
This is not the correct way to submit patches for inclusion in the
stable kernel tree. Please read Documentation/stable_kernel_rules.txt
for how to do this properly.
</formletter>
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