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| H A D | iothread.c | 6c95363d Tue Jan 29 05:14:32 GMT 2019 Peter Xu <peterx@redhat.com> iothread: fix iothread hang when stop too soon
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 <thuth@redhat.com>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Lukáš Doktor <ldoktor@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Lukáš Doktor <ldoktor@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-id: 20190129051432.22023-1-peterx@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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