Re: [Qemu-devel] [PATCH for-2.9-rc5 v4 1/2] block: Walk bs->children carefully in bdrv_drain_recurse

[Fam Zheng <famz@redhat.com>]

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 <famz@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