[PATCH] block: Fix deadlock in bdrv_set_aio_context_ignore()

[Zhiyong Ye <yezhiyong@bytedance.com>]

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.

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
...

[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 <yezhiyong@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);
 }
 
 static bool bdrv_parent_can_set_aio_context(BdrvChild *c, AioContext *ctx,
-- 
2.11.0