Re: dm: don't reuse bio for flushes

[Mike Snitzer <snitzer@redhat.com>]

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>