On Wed, Dec 19, 2018 at 09:14:25AM -0700, Jens Axboe wrote:
> On 12/19/18 9:13 AM, Mike Snitzer wrote:
> > On Wed, Dec 19 2018 at 11:11am -0500,
> > Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> >
> >> On Wed, Dec 19 2018 at 10:50am -0500,
> >> Jens Axboe <axboe@xxxxxxxxx> 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@xxxxxxxxxx>
> >>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> >>
> >> 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@xxxxxxxxxx>
> >
> > Please upgrade this to:
> >
> > Reviewed-by: Mike Snitzer <snitzer@xxxxxxxxxx>
>
> Done, thanks Mike.
>
> --
> Jens Axboe
>
Thanks all of you for helping fix this issue! Apologies for introducing
with the initial change to dm.
Thanks,
Dennis
