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block: Walk bs->children carefully in bdrv_drain_recurse

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The recursive bdrv_drain_recurse may run a block job completion BH that
drops nodes. The coming changes will make that more likely and use-after-free
would happen without this patch

Stash the bs pointer and use bdrv_ref/bdrv_unref in addition to
QLIST_FOREACH_SAFE to prevent such a case from happening.

Since bdrv_unref accesses global state that is not protected by the AioContext
lock, we cannot use bdrv_ref/bdrv_unref unconditionally.  Fortunately the
protection is not needed in IOThread because only main loop can modify a graph
with the AioContext lock held.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170418143044.12187-2-famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Tested-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
This commit is contained in:
Fam Zheng 2017-04-18 22:30:43 +08:00
parent 9c6b899f7a
commit 178bd438af
1 changed files with 20 additions and 3 deletions
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23
block/io.c
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@ -158,7 +158,7 @@ bool bdrv_requests_pending(BlockDriverState *bs)
static bool bdrv_drain_recurse(BlockDriverState *bs) static bool bdrv_drain_recurse(BlockDriverState *bs)
{ {
BdrvChild *child; BdrvChild *child, *tmp;
bool waited; bool waited;
waited = BDRV_POLL_WHILE(bs, atomic_read(&bs->in_flight) > 0); waited = BDRV_POLL_WHILE(bs, atomic_read(&bs->in_flight) > 0);
@ -167,8 +167,25 @@ static bool bdrv_drain_recurse(BlockDriverState *bs)
bs->drv->bdrv_drain(bs); bs->drv->bdrv_drain(bs);
} }
QLIST_FOREACH(child, &bs->children, next) { QLIST_FOREACH_SAFE(child, &bs->children, next, tmp) {
waited |= bdrv_drain_recurse(child->bs); BlockDriverState *bs = child->bs;
bool in_main_loop =
qemu_get_current_aio_context() == qemu_get_aio_context();
assert(bs->refcnt > 0);
if (in_main_loop) {
/* In case the recursive bdrv_drain_recurse processes a
* block_job_defer_to_main_loop BH and modifies the graph,
* let's hold a reference to bs until we are done.
*
* IOThread doesn't have such a BH, and it is not safe to call
* bdrv_unref without BQL, so skip doing it there.
*/
bdrv_ref(bs);
}
waited |= bdrv_drain_recurse(bs);
if (in_main_loop) {
bdrv_unref(bs);
}
} }
return waited; return waited;