| Message ID | 20190114133257.30299-2-stefanha@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | throttle-groups: fix restart coroutine iothread race | expand |
On Mon 14 Jan 2019 02:32:56 PM CET, Stefan Hajnoczi wrote: > The following QMP command leads to a crash when iothreads are used: > > { 'execute': 'device_del', 'arguments': {'id': 'data'} } > > The backtrace involves the queue restart coroutine where > tgm->throttle_state is a NULL pointer because > throttle_group_unregister_tgm() has already been called: > > (gdb) bt full > #0 0x00005585a7a3b378 in qemu_mutex_lock_impl (mutex=0xffffffffffffffd0, file=0x5585a7bb3d54 "block/throttle-groups.c", line=412) at util/qemu-thread-posix.c:64 > err = <optimized out> > __PRETTY_FUNCTION__ = "qemu_mutex_lock_impl" > __func__ = "qemu_mutex_lock_impl" > #1 0x00005585a79be074 in throttle_group_restart_queue_entry (opaque=0x5585a9de4eb0) at block/throttle-groups.c:412 > _f = <optimized out> > data = 0x5585a9de4eb0 > tgm = 0x5585a9079440 > ts = 0x0 > tg = 0xffffffffffffff98 > is_write = false > empty_queue = 255 > > This coroutine should not execute in the iothread after the throttle > group member has been unregistered! > > The root cause is that the device_del code path schedules the restart > coroutine in the iothread while holding the AioContext lock. Therefore > the iothread cannot execute the coroutine until after device_del > releases the lock - by this time it's too late. > > This patch adds a reference count to ThrottleGroupMember so we can > synchronously wait for restart coroutines to complete. Once they are > done it is safe to unregister the ThrottleGroupMember. > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Berto
diff --git a/include/block/throttle-groups.h b/include/block/throttle-groups.h index e2fd0513c4..712a8e64b4 100644 --- a/include/block/throttle-groups.h +++ b/include/block/throttle-groups.h @@ -43,6 +43,11 @@ typedef struct ThrottleGroupMember { */ unsigned int io_limits_disabled; + /* Number of pending throttle_group_restart_queue_entry() coroutines. + * Accessed with atomic operations. + */ + unsigned int restart_pending; + /* The following fields are protected by the ThrottleGroup lock. * See the ThrottleGroup documentation for details. * throttle_state tells us if I/O limits are configured. */ diff --git a/block/throttle-groups.c b/block/throttle-groups.c index 5d8213a443..a5a2037924 100644 --- a/block/throttle-groups.c +++ b/block/throttle-groups.c @@ -415,6 +415,9 @@ static void coroutine_fn throttle_group_restart_queue_entry(void *opaque) } g_free(data); + + atomic_dec(&tgm->restart_pending); + aio_wait_kick(); } static void throttle_group_restart_queue(ThrottleGroupMember *tgm, bool is_write) @@ -430,6 +433,8 @@ static void throttle_group_restart_queue(ThrottleGroupMember *tgm, bool is_write * be no timer pending on this tgm at this point */ assert(!timer_pending(tgm->throttle_timers.timers[is_write])); + atomic_inc(&tgm->restart_pending); + co = qemu_coroutine_create(throttle_group_restart_queue_entry, rd); aio_co_enter(tgm->aio_context, co); } @@ -538,6 +543,7 @@ void throttle_group_register_tgm(ThrottleGroupMember *tgm, tgm->throttle_state = ts; tgm->aio_context = ctx; + atomic_set(&tgm->restart_pending, 0); qemu_mutex_lock(&tg->lock); /* If the ThrottleGroup is new set this ThrottleGroupMember as the token */ @@ -584,6 +590,9 @@ void throttle_group_unregister_tgm(ThrottleGroupMember *tgm) return; } + /* Wait for throttle_group_restart_queue_entry() coroutines to finish */ + AIO_WAIT_WHILE(tgm->aio_context, atomic_read(&tgm->restart_pending) > 0); + qemu_mutex_lock(&tg->lock); for (i = 0; i < 2; i++) { assert(tgm->pending_reqs[i] == 0);
The following QMP command leads to a crash when iothreads are used: { 'execute': 'device_del', 'arguments': {'id': 'data'} } The backtrace involves the queue restart coroutine where tgm->throttle_state is a NULL pointer because throttle_group_unregister_tgm() has already been called: (gdb) bt full #0 0x00005585a7a3b378 in qemu_mutex_lock_impl (mutex=0xffffffffffffffd0, file=0x5585a7bb3d54 "block/throttle-groups.c", line=412) at util/qemu-thread-posix.c:64 err = <optimized out> __PRETTY_FUNCTION__ = "qemu_mutex_lock_impl" __func__ = "qemu_mutex_lock_impl" #1 0x00005585a79be074 in throttle_group_restart_queue_entry (opaque=0x5585a9de4eb0) at block/throttle-groups.c:412 _f = <optimized out> data = 0x5585a9de4eb0 tgm = 0x5585a9079440 ts = 0x0 tg = 0xffffffffffffff98 is_write = false empty_queue = 255 This coroutine should not execute in the iothread after the throttle group member has been unregistered! The root cause is that the device_del code path schedules the restart coroutine in the iothread while holding the AioContext lock. Therefore the iothread cannot execute the coroutine until after device_del releases the lock - by this time it's too late. This patch adds a reference count to ThrottleGroupMember so we can synchronously wait for restart coroutines to complete. Once they are done it is safe to unregister the ThrottleGroupMember. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> --- v2: * Use atomic operations instead of AioContext lock [Paolo] --- include/block/throttle-groups.h | 5 +++++ block/throttle-groups.c | 9 +++++++++ 2 files changed, 14 insertions(+)