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authorFam Zheng <famz@redhat.com>2017-04-18 22:30:43 +0800
committerFam Zheng <famz@redhat.com>2017-04-18 22:56:28 +0800
commit178bd438af5c95deef5073416c60396f88e97ec9 (patch)
treee6637a6b80a892746126e6270dc009466fc64885
parent9c6b899f7a46893ab3b671e341a2234e9c0c060e (diff)
downloadqemu-arm-178bd438af5c95deef5073416c60396f88e97ec9.tar.gz
block: Walk bs->children carefully in bdrv_drain_recurse
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>
-rw-r--r--block/io.c23
1 files changed, 20 insertions, 3 deletions
diff --git a/block/io.c b/block/io.c
index 8706bfa578..a7142e00e8 100644
--- a/block/io.c
+++ b/block/io.c
@@ -158,7 +158,7 @@ bool bdrv_requests_pending(BlockDriverState *bs)
static bool bdrv_drain_recurse(BlockDriverState *bs)
{
- BdrvChild *child;
+ BdrvChild *child, *tmp;
bool waited;
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);
}
- QLIST_FOREACH(child, &bs->children, next) {
- waited |= bdrv_drain_recurse(child->bs);
+ QLIST_FOREACH_SAFE(child, &bs->children, next, tmp) {
+ 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;