Am 18.04.2017 um 16:30 hat Fam Zheng geschrieben:
> 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 <f...@redhat.com>
> ---
> block/io.c | 23 ++++++++++++++++++++---
> 1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/block/io.c b/block/io.c
> index 8706bfa..a0df8c4 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 resursive bdrv_drain_recurse processes a
s/resursive/recursive/
> + * 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.
> + **/
And **/ is unusual, too.
> + bdrv_ref(bs);
> + }
> + waited |= bdrv_drain_recurse(bs);
> + if (in_main_loop) {
> + bdrv_unref(bs);
> + }
> }
Other than this, the series looks good to me.
Kevin
