From 2e5b887cfc69991eee27be6cc0938c70a360fe45 Mon Sep 17 00:00:00 2001 From: Fam Zheng Date: Sun, 15 Feb 2015 11:06:30 +0800 Subject: [PATCH] block: Forbid bdrv_set_aio_context outside BQL Even if the caller has both the old and the new AioContext's, there can be a deadlock, due to the leading bdrv_drain_all. Suppose there are four io threads (A, B, A0, B0) with A and B owning a BDS for each (bs_a, bs_b); Now A wants to move bs_a to iothread A0, and B wants to move bs_b to B0, at the same time: iothread A iothread B -------------------------------------------------------------------------- aio_context_acquire(A0) /* OK */ aio_context_acquire(B0) /* OK */ bdrv_set_aio_context(bs_a, A0) bdrv_set_aio_context(bs_b, B0) -> bdrv_drain_all() -> bdrv_drain_all() -> acquire A /* OK */ -> acquire A /* blocked */ -> acquire B /* blocked */ -> acquire B ... ... Deadlock happens because A is waiting for B, and B is waiting for A. Signed-off-by: Fam Zheng Reviewed-by: Paolo Bonzini Message-Id: <1423969591-23646-2-git-send-email-famz@redhat.com> Signed-off-by: Paolo Bonzini --- include/block/block.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/include/block/block.h b/include/block/block.h index 471d11dbbe..649c269ecd 100644 --- a/include/block/block.h +++ b/include/block/block.h @@ -547,8 +547,7 @@ AioContext *bdrv_get_aio_context(BlockDriverState *bs); * Changes the #AioContext used for fd handlers, timers, and BHs by this * BlockDriverState and all its children. * - * This function must be called from the old #AioContext or with a lock held so - * the old #AioContext is not executing. + * This function must be called with iothread lock held. */ void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context);