Re: [PATCH v4] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim

Dave Chinner
2020-06-18T23:04:35Z

On Fri, Jun 19, 2020 at 08:58:10AM +1000, Dave Chinner wrote:
> On Thu, Jun 18, 2020 at 01:19:41PM -0400, Waiman Long wrote:
> > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > index 379cbff438bc..1b94b9bfa4d7 100644
> > --- a/fs/xfs/xfs_super.c
> > +++ b/fs/xfs/xfs_super.c
> > @@ -913,11 +913,33 @@ xfs_fs_freeze(
> > struct super_block *sb)
> > {
> > struct xfs_mount *mp = XFS_M(sb);
> > + unsigned long pflags;
> > + int ret;
> >
> > + /*
> > + * A fs_reclaim pseudo lock is added to check for potential deadlock
> > + * condition with fs reclaim. The following lockdep splat was hit
> > + * occasionally. This is actually a false positive as the allocation
> > + * is being done only after the frozen filesystem is no longer dirty.
> > + * One way to avoid this splat is to add GFP_NOFS to the affected
> > + * allocation calls. This is what PF_MEMALLOC_NOFS is for.
> > + *
> > + * CPU0 CPU1
> > + * ---- ----
> > + * lock(sb_internal);
> > + * lock(fs_reclaim);
> > + * lock(sb_internal);
> > + * lock(fs_reclaim);
> > + *
> > + * *** DEADLOCK ***
> > + */
>
> The lockdep splat is detailed in the commit message - it most
> definitely does not need to be repeated in full here because:
>
> a) it doesn't explain why the splat occurring is, and
> b) we most definitely don't care about how the lockdep check
> that triggered it is implemented.

I should have added this:

c) a lot of people don't understand what lockdep reports
are telling them is a problem.

I get a lot of questions like "I saw this lockdep thing, but I can't
work out what it actually means, so can you have a look at it
Dave?". Hence I think directly quoting something people tend not to
understand to explain the problem they didn't understand isn't the
best approach to improving understanding of the problem...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com