Re: [PATCH] block: temporarily hold the new AioContext of bs_top in bdrv_append()

[Stefano Garzarella]

On Tue, Feb 14, 2023 at 12:56 PM Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 14.02.2023 um 11:51 hat Stefano Garzarella geschrieben:
> > bdrv_append() is called with bs_top AioContext held, but
> > bdrv_attach_child_noperm() could change the AioContext of bs_top.
> >
> > bdrv_replace_node_noperm() calls bdrv_drained_begin() starting from
> > commit 2398747128 ("block: Don't poll in bdrv_replace_child_noperm()").
> > bdrv_drained_begin() can call BDRV_POLL_WHILE that assumes the new lock
> > is taken, so let's temporarily hold the new AioContext to prevent QEMU
> > from failing in BDRV_POLL_WHILE when it tries to release the wrong
> > AioContext.
> >
> > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2168209
> > Reported-by: Aihua Liang <aliang@redhat.com>
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > ---
> > I'm not sure whether to use the following Fixes tag. That commit added the
> > calls to bdrv_drained_begin() in bdrv_replace_node_noperm(), but maybe the
> > problem was pre-existing.
> >
> > Fixes: 2398747128 ("block: Don't poll in bdrv_replace_child_noperm()")
> >
> > Note: a local reproducer is attached in the BZ, it is based on the Aihua 
> > Liang
> > report and it hits the issue with a 20% ratio.
> > ---
> >  block.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> >
> > diff --git a/block.c b/block.c
> > index aa9062f2c1..0e2bc11e0b 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -5266,6 +5266,8 @@ int bdrv_drop_filter(BlockDriverState *bs, Error 
> > **errp)
> >   * child.
> >   *
> >   * This function does not create any image files.
> > + *
> > + * The caller must hold the AioContext lock for @bs_top.
> >   */
> >  int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
> >                  Error **errp)
> > @@ -5273,11 +5275,14 @@ int bdrv_append(BlockDriverState *bs_new, 
> > BlockDriverState *bs_top,
> >      int ret;
> >      BdrvChild *child;
> >      Transaction *tran = tran_new();
> > +    AioContext *old_context, *new_context;
> >
> >      GLOBAL_STATE_CODE();
> >
> >      assert(!bs_new->backing);
> >
> > +    old_context = bdrv_get_aio_context(bs_top);
> > +
> >      child = bdrv_attach_child_noperm(bs_new, bs_top, "backing",
> >                                       &child_of_bds, 
> > bdrv_backing_role(bs_new),
> >                                       tran, errp);
> > @@ -5286,11 +5291,29 @@ int bdrv_append(BlockDriverState *bs_new, 
> > BlockDriverState *bs_top,
> >          goto out;
> >      }
> >
> > +    /*
> > +     * bdrv_attach_child_noperm could change the AioContext of bs_top.
> > +     * bdrv_replace_node_noperm calls bdrv_drained_begin, so let's 
> > temporarily
> > +     * hold the new AioContext, since bdrv_drained_begin calls 
> > BDRV_POLL_WHILE
> > +     * that assumes the new lock is taken.
> > +     */
> > +    new_context = bdrv_get_aio_context(bs_top);
> > +
> > +    if (old_context != new_context) {
> > +        aio_context_release(old_context);
> > +        aio_context_acquire(new_context);
> > +    }
> > +
> >      ret = bdrv_replace_node_noperm(bs_top, bs_new, true, tran, errp);
> >      if (ret < 0) {
> >          goto out;
>
> If we take the error path, we return with new_context locked instead of
> old_context now.

Grr, I'm blind...

>
> >      }
> >
> > +    if (old_context != new_context) {
> > +        aio_context_release(new_context);
> > +        aio_context_acquire(old_context);
> > +    }
> > +
> >      ret = bdrv_refresh_perms(bs_new, tran, errp);
> >  out:
> >      tran_finalize(tran, ret);
>
> Strictly speaking, don't we need to hold the lock across
> tran_finalize(), too? It completes the bdrv_replace_node_noperm() call
> you covered above.

Right!

>
> Maybe bdrv_refresh_perms() and bdrv_refresh_limits(), too, in fact. We
> never clearly defined which functions need the lock and which don't, so
> hard to tell.

Okay, so to be on the safe side, I'll switch them back just before return.

> It's really time to get rid of it.

How could one disagree? :-)

What about the Fixes tag? Should I include it?

Thanks,
Stefano