Re: dm: don't reuse bio for flushes

On Wed, Dec 19 2018 at 11:11am -0500,
Mike Snitzer [off-list ref] wrote:

On Wed, Dec 19 2018 at 10:50am -0500,
Jens Axboe [off-list ref] wrote:

quoted

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 <redacted>
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 <redacted>

Please upgrade this to:

Reviewed-by: Mike Snitzer <redacted>