From 40840e419be31e6a32e6ea24511c74b389d5e0e4 Mon Sep 17 00:00:00 2001 From: Alberto Garcia Date: Fri, 28 Oct 2016 10:08:03 +0300 Subject: [PATCH] block: Pause all jobs during bdrv_reopen_multiple() When a BlockDriverState is about to be reopened it can trigger certain operations that need to write to disk. During this process a different block job can be woken up. If that block job completes and also needs to call bdrv_reopen() it can happen that it needs to do it on the same BlockDriverState that is still in the process of being reopened. This can have fatal consequences, like in this example: 1) Block job A starts and sleeps after a while. 2) Block job B starts and tries to reopen node1 (a qcow2 file). 3) Reopening node1 means flushing and replacing its qcow2 cache. 4) While the qcow2 cache is being flushed, job A wakes up. 5) Job A completes and reopens node1, replacing its cache. 6) Job B resumes, but the cache that was being flushed no longer exists. This patch splits the bdrv_drain_all() call to keep all block jobs paused during bdrv_reopen_multiple(), so that step 4 can never happen and the operation is safe. Note that this scenario can only happen if both bdrv_reopen() calls are made by block jobs on the same backing chain. Otherwise there's no chance that the same BlockDriverState appears in both reopen queues. Signed-off-by: Alberto Garcia Reviewed-by: Paolo Bonzini Reviewed-by: Kevin Wolf Signed-off-by: Kevin Wolf --- block.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/block.c b/block.c index a17baab1d0a3..2c87186ecfcc 100644 --- a/block.c +++ b/block.c @@ -2091,7 +2091,7 @@ int bdrv_reopen_multiple(AioContext *ctx, BlockReopenQueue *bs_queue, Error **er assert(bs_queue != NULL); aio_context_release(ctx); - bdrv_drain_all(); + bdrv_drain_all_begin(); aio_context_acquire(ctx); QSIMPLEQ_FOREACH(bs_entry, bs_queue, entry) { @@ -2122,6 +2122,9 @@ int bdrv_reopen_multiple(AioContext *ctx, BlockReopenQueue *bs_queue, Error **er g_free(bs_entry); } g_free(bs_queue); + + bdrv_drain_all_end(); + return ret; }