Simimlar to AIO_WAIT_WHILE(), bdrv_drain_poll_top_level() needs to release the AioContext lock of the node to be drained before calling aio_poll(). Otherwise, callbacks called by aio_poll() would possibly take the lock a second time and run into a deadlock with a nested AIO_WAIT_WHILE() call.
Signed-off-by: Kevin Wolf <kw...@redhat.com> --- block/io.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) diff --git a/block/io.c b/block/io.c index 7100344c7b..832d2536bf 100644 --- a/block/io.c +++ b/block/io.c @@ -268,9 +268,32 @@ bool bdrv_drain_poll(BlockDriverState *bs, bool recursive, static bool bdrv_drain_poll_top_level(BlockDriverState *bs, bool recursive, BdrvChild *ignore_parent) { + AioContext *ctx = bdrv_get_aio_context(bs); + + /* + * We cannot easily release the lock unconditionally here because many + * callers of drain function (like qemu initialisation, tools, etc.) don't + * even hold the main context lock. + * + * This means that we fix potential deadlocks for the case where we are in + * the main context and polling a BDS in a different AioContext, but + * draining a BDS in the main context from a different I/O thread would + * still have this problem. Fortunately, this isn't supposed to happen + * anyway. + */ + if (ctx != qemu_get_aio_context()) { + aio_context_release(ctx); + } else { + assert(qemu_get_current_aio_context() == qemu_get_aio_context()); + } + /* Execute pending BHs first and check everything else only after the BHs * have executed. */ - while (aio_poll(bs->aio_context, false)); + while (aio_poll(ctx, false)); + + if (ctx != qemu_get_aio_context()) { + aio_context_acquire(ctx); + } return bdrv_drain_poll(bs, recursive, ignore_parent, false); } -- 2.13.6