On Tue, 04/18 16:46, Kevin Wolf wrote:
> 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.
Thanks, I'll fix them and send a pull request to Peter.
Fam
