Am 12.07.2021 um 07:38 hat Zhiyong Ye geschrieben: > When bdrv_set_aio_context_ignore() is called in the main loop to change > the AioContext onto the IO thread, the bdrv_drain_invoke_entry() never > gets to run and the IO thread hangs at co_schedule_bh_cb(). > > This is because the AioContext is occupied by the main thread after > being attached to the IO thread, and the main thread poll in > bdrv_drained_end() waiting for the IO request to be drained, but the IO > thread cannot acquire the AioContext, which leads to deadlock.
This shouldn't be the case: * The caller must own the AioContext lock for the old AioContext of bs, but it * must not own the AioContext lock for new_context (unless new_context is the * same as the current context of bs). Then bdrv_set_aio_context_ignore() switches the AioContext of bs, and then calls bdrv_drained_end() while holding only the lock for the new context. AIO_WAIT_WHILE() will temporarily drop that lock, so aio_poll() should run without holding any AioContext locks. If I'm not missing anything, the scenario you're seeing means that the caller already held a lock for the new AioContext, so that it's locked twice while AIO_WAIT_WHILE() drops the lock only once. This would be a bug in the caller because the documentation I quoted explicitly forbids holding the AioContext lock for the new context. > Just like below: > > <------> > [Switching to thread 1 (Thread 0x7fd810bbef40 (LWP 533312))] > (gdb) bt > ... > 3 0x00005601f6ea93aa in fdmon_poll_wait at ../util/fdmon-poll.c:80 > 4 0x00005601f6e81a1c in aio_poll at ../util/aio-posix.c:607 > 5 0x00005601f6dcde87 in bdrv_drained_end at ../block/io.c:496 > 6 0x00005601f6d798cd in bdrv_set_aio_context_ignore at ../block.c:6502 > 7 0x00005601f6d7996c in bdrv_set_aio_context_ignore at ../block.c:6472 > 8 0x00005601f6d79cb8 in bdrv_child_try_set_aio_context at ../block.c:6587 > 9 0x00005601f6da86f2 in blk_do_set_aio_context at > ../block/block-backend.c:2026 > 10 0x00005601f6daa96d in blk_set_aio_context at ../block/block-backend.c:2047 > 11 0x00005601f6c71883 in virtio_scsi_hotplug at ../hw/scsi/virtio-scsi.c:831 Which version of QEMU are you using? In current git master, line 831 is something entirely different. Are you using something before commit c7040ff6? Because this is a commit that fixed a virtio-scsi bug where it would hold the wrong lock before calling blk_set_aio_context(). > > [Switching to thread 4 (Thread 0x7fd8092e7700 (LWP 533315))] > (gdb) bt > ... > 4 0x00005601f6eab6a8 in qemu_mutex_lock_impl at > ../util/qemu-thread-posix.c:79 > 5 0x00005601f6e7ce88 in co_schedule_bh_cb at ../util/async.c:489 > 6 0x00005601f6e7c404 in aio_bh_poll at ../util/async.c:164 > 7 0x00005601f6e81a46 in aio_poll at ../util/aio-posix.c:659 > 8 0x00005601f6d5ccf3 in iothread_run at ../iothread.c:73 > 9 0x00005601f6eab512 in qemu_thread_start at ../util/qemu-thread-posix.c:521 > 10 0x00007fd80d7b84a4 in start_thread at pthread_create.c:456 > 11 0x00007fd80d4fad0f in clone () at > ../sysdeps/unix/sysv/linux/x86_64/clone.S:97 > (gdb) f 4 > 4 0x00005601f6eab6a8 in qemu_mutex_lock_impl at > ../util/qemu-thread-posix.c:79 > (gdb) p *mutex > $2 = {lock = {__data = {__lock = 2, __count = 1, __owner = 533312, __nusers = > 1, > __kind = 1, __spins = 0, __elision = 0, __list = {__prev = 0x0, __next > = 0x0}}, > __size = "\002\000\000\000\001\000\000\000@#\b\000\001\000\000\000\001", > '\000' <repeats 22 times>, __align = 4294967298}, initialized = true} > <------> > > Therefore, we should never poll anywhere in > bdrv_set_aio_context_ignore() when acquiring the new context. In fact, > commit e037c09c has also already elaborated on why we can't poll at > bdrv_do_drained_end(). > > Signed-off-by: Zhiyong Ye <yezhiy...@bytedance.com> > --- > block.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/block.c b/block.c > index be083f389e..ebbea72d64 100644 > --- a/block.c > +++ b/block.c > @@ -6846,6 +6846,7 @@ void bdrv_set_aio_context_ignore(BlockDriverState *bs, > GSList *parents_to_process = NULL; > GSList *entry; > BdrvChild *child, *parent; > + int drained_end_counter = 0; > > g_assert(qemu_get_current_aio_context() == qemu_get_aio_context()); > > @@ -6907,7 +6908,7 @@ void bdrv_set_aio_context_ignore(BlockDriverState *bs, > aio_context_release(old_context); > } > > - bdrv_drained_end(bs); > + bdrv_drained_end_no_poll(bs, &drained_end_counter); > > if (qemu_get_aio_context() != old_context) { > aio_context_acquire(old_context); > @@ -6915,6 +6916,7 @@ void bdrv_set_aio_context_ignore(BlockDriverState *bs, > if (qemu_get_aio_context() != new_context) { > aio_context_release(new_context); > } > + BDRV_POLL_WHILE(bs, qatomic_read(&drained_end_counter) > 0); This would be wrong because bs is already in the new context, but you wouldn't hold the lock for it. AIO_WAIT_WHILE() would try to drop the lock for a context that isn't even locked, resulting in a crash. > } > > static bool bdrv_parent_can_set_aio_context(BdrvChild *c, AioContext *ctx, Kevin