linux-kernel - Re: [PATCH v4 04/11] md/raid5: Ensure array is suspended for calls to log

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Date:   Wed, 8 Jun 2022 15:02:46 -0700
From:   Song Liu <song@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        linux-raid <linux-raid@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Donald Buczek <buczek@...gen.mpg.de>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Xiao Ni <xni@...hat.com>, Stephen Bates <sbates@...thlin.com>,
        Martin Oliveira <Martin.Oliveira@...eticom.com>,
        David Sloan <David.Sloan@...eticom.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 04/11] md/raid5: Ensure array is suspended for calls to log_exit()

On Wed, Jun 8, 2022 at 11:21 AM Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
>
> On 2022-06-08 11:59, Song Liu wrote:
> > On Wed, Jun 8, 2022 at 9:28 AM Logan Gunthorpe <logang@...tatee.com> wrote:
> >>
> >> The raid5-cache code relies on there being no IO in flight when
> >> log_exit() is called. There are two places where this is not
> >> guaranteed so add mddev_suspend() and mddev_resume() calls to these
> >> sites.
> >>
> >> The site in raid5_remove_disk() has a comment saying that it is
> >> called in raid5d and thus cannot wait for pending writes; however that
> >> does not appear to be correct anymore (if it ever was) as
> >> raid5_remove_disk() is called from hot_remove_disk() which only
> >> appears to be called in the md_ioctl(). Thus, the comment is removed,
> >> as well as the racy check and replaced with calls to suspend/resume.
> >>
> >> The site in raid5_change_consistency_policy() is in the error path,
> >> and another similar call site already has suspend/resume calls just
> >> below it; so it should be equally safe to make that change here.
> >>
> >> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> >> Reviewed-by: Christoph Hellwig <hch@....de>
> >> ---
> >>  drivers/md/raid5.c | 18 ++++++------------
> >>  1 file changed, 6 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> >> index 5d09256d7f81..3ad37dd4c5cd 100644
> >> --- a/drivers/md/raid5.c
> >> +++ b/drivers/md/raid5.c
> >> @@ -7938,18 +7938,9 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
> >>
> >>         print_raid5_conf(conf);
> >>         if (test_bit(Journal, &rdev->flags) && conf->log) {
> >> -               /*
> >> -                * we can't wait pending write here, as this is called in
> >> -                * raid5d, wait will deadlock.
> >> -                * neilb: there is no locking about new writes here,
> >> -                * so this cannot be safe.
> >> -                */
> >> -               if (atomic_read(&conf->active_stripes) ||
> >> -                   atomic_read(&conf->r5c_cached_full_stripes) ||
> >> -                   atomic_read(&conf->r5c_cached_partial_stripes)) {
> >> -                       return -EBUSY;
> >> -               }
> >> +               mddev_suspend(mddev);
> >
> > Unfortunately, the comment about deadlock is still true, and we cannot call
> > mddev_suspend here. To trigger it:
>
> Ah, yes. What a tangle. I think we can just drop this patch. Now that we
> are removing RCU it isn't actually necessary to fix the bug I was
> seeing. It's still probably broken as the comment notes though.

How about we keep the suspend/resume in raid5_change_consistency_policy()?
Like the one I just pushed to md-next:

https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?h=md-next&id=ac1506992459fe45a085c1f93df74d51c381887b

Thanks,
Song