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[Qemu-devel] [PULL 1/2] block: Walk bs->children carefully in bdrv_drain
From: |
Fam Zheng |
Subject: |
[Qemu-devel] [PULL 1/2] block: Walk bs->children carefully in bdrv_drain_recurse |
Date: |
Tue, 18 Apr 2017 23:01:32 +0800 |
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 <address@hidden>
Message-Id: <address@hidden>
Reviewed-by: Jeff Cody <address@hidden>
Tested-by: Jeff Cody <address@hidden>
Signed-off-by: Fam Zheng <address@hidden>
---
block/io.c | 23 ++++++++++++++++++++---
1 file changed, 20 insertions(+), 3 deletions(-)
diff --git a/block/io.c b/block/io.c
index 8706bfa..a7142e0 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;
--
2.9.3