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author | Fam Zheng <famz@redhat.com> | 2015-02-15 11:06:30 +0800 |
---|---|---|
committer | Paolo Bonzini <pbonzini@redhat.com> | 2015-02-27 14:43:45 +0100 |
commit | 2e5b887cfc69991eee27be6cc0938c70a360fe45 (patch) | |
tree | 7aca623f1700e7fe3bc5fd9c344930eaa73b71dc /include | |
parent | 2264750483107c45877d29813c497b4c87f64cb6 (diff) | |
download | hqemu-2e5b887cfc69991eee27be6cc0938c70a360fe45.zip hqemu-2e5b887cfc69991eee27be6cc0938c70a360fe45.tar.gz |
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 <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1423969591-23646-2-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'include')
-rw-r--r-- | include/block/block.h | 3 |
1 files changed, 1 insertions, 2 deletions
diff --git a/include/block/block.h b/include/block/block.h index 471d11d..649c269 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); |