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authorFam Zheng <famz@redhat.com>2015-02-15 11:06:30 +0800
committerPaolo Bonzini <pbonzini@redhat.com>2015-02-27 14:43:45 +0100
commit2e5b887cfc69991eee27be6cc0938c70a360fe45 (patch)
tree7aca623f1700e7fe3bc5fd9c344930eaa73b71dc /hw/scsi
parent2264750483107c45877d29813c497b4c87f64cb6 (diff)
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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>
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