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Re: [Qemu-devel] [PATCH] iothread: fix iothread hang when stop too soon
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH] iothread: fix iothread hang when stop too soon |
Date: |
Wed, 30 Jan 2019 11:28:07 +0800 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
On Tue, Jan 29, 2019 at 01:14:32PM +0800, Peter Xu wrote:
> Lukas reported an hard to reproduce QMP iothread hang on s390 that
> QEMU might hang at pthread_join() of the QMP monitor iothread before
> quitting:
>
> Thread 1
> #0 0x000003ffad10932c in pthread_join
> #1 0x0000000109e95750 in qemu_thread_join
> at /home/thuth/devel/qemu/util/qemu-thread-posix.c:570
> #2 0x0000000109c95a1c in iothread_stop
> #3 0x0000000109bb0874 in monitor_cleanup
> #4 0x0000000109b55042 in main
>
> While the iothread is still in the main loop:
>
> Thread 4
> #0 0x000003ffad0010e4 in ??
> #1 0x000003ffad553958 in g_main_context_iterate.isra.19
> #2 0x000003ffad553d90 in g_main_loop_run
> #3 0x0000000109c9585a in iothread_run
> at /home/thuth/devel/qemu/iothread.c:74
> #4 0x0000000109e94752 in qemu_thread_start
> at /home/thuth/devel/qemu/util/qemu-thread-posix.c:502
> #5 0x000003ffad10825a in start_thread
> #6 0x000003ffad00dcf2 in thread_start
>
> IMHO it's because there's a race between the main thread and iothread
> when stopping the thread in following sequence:
>
> main thread iothread
> =========== ==============
> aio_poll()
> iothread_get_g_main_context
> set iothread->worker_context
> iothread_stop
> schedule iothread_stop_bh
> execute iothread_stop_bh [1]
> set iothread->running=false
> (since main_loop==NULL so
> skip to quit main loop.
> Note: although main_loop is
> NULL but worker_context is
> not!)
> atomic_read(&iothread->worker_context)
> [2]
> create main_loop object
> g_main_loop_run() [3]
> pthread_join() [4]
>
> We can see that when execute iothread_stop_bh() at [1] it's possible
> that main_loop is still NULL because it's only created until the first
> check of the worker_context later at [2]. Then the iothread will hang
> in the main loop [3] and it'll starve the main thread too [4].
>
> Here the simple solution should be that we check again the "running"
> variable before check against worker_context.
>
> CC: Thomas Huth <address@hidden>
> CC: Dr. David Alan Gilbert <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Lukáš Doktor <address@hidden>
> CC: Markus Armbruster <address@hidden>
> CC: Eric Blake <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> Reported-by: Lukáš Doktor <address@hidden>
> Signed-off-by: Peter Xu <address@hidden>
> ---
>
> This hasn't yet been verified on the initial s390 systems, but since I
> can reproduce it locally with this code clip:
>
> IOThread *iothread = iothread_create("test", NULL);
> iothread_get_g_main_context(iothread);
> iothread_stop(iothread);
>
> so I'm still posting this out for review first in case it was hit by
> other users.
> ---
> iothread.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block
Stefan
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