Re: dm: don't reuse bio for flushes

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Dec 19 2018 at 11:11am -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Wed, Dec 19 2018 at 10:50am -0500,
> Jens Axboe <axboe@kernel.dk> wrote:
> 
> > DM currently has a statically allocated bio that it uses to issue empty
> > flushes. It doesn't submit this bio, it just uses it for maintaining
> > state while setting up clones. Multiple users can access this bio at the
> > same time. This wasn't previously an issue, even if it was a bit iffy,
> > but with the blkg associations it can become one.
> > 
> > We setup the blkg association, then clone bio's and submit, then remove
> > the blkg assocation again. But since we can have multiple tasks doing
> > this at the same time, against multiple blkg's, then we can either lose
> > references to a blkg, or put it twice. The latter causes complaints on
> > the percpu ref being <= 0 when released, and can cause use-after-free as
> > well. Ming reports that xfstest generic/475 triggers this:
> > 
> > ------------[ cut here ]------------
> > percpu ref (blkg_release) <= 0 (0) after switching to atomic
> > WARNING: CPU: 13 PID: 0 at lib/percpu-refcount.c:155 percpu_ref_switch_to_atomic_rcu+0x2c9/0x4a0
> > 
> > Switch to just using an on-stack bio for this, and get rid of the
> > embedded bio.
> > 
> > Fixes: 5cdf2e3fea5e ("blkcg: associate blkg when associating a device")
> > Reported-by: Ming Lei <ming.lei@redhat.com>
> > Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> Thanks for sorting this one out, definitely wasn't happy with how
> exposed DM was left with the recent blkg changes.  This is clearly
> better.
> 
> Acked-by: Mike Snitzer <snitzer@redhat.com>

Please upgrade this to:

Reviewed-by: Mike Snitzer <snitzer@redhat.com>