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

[Dave Chinner <david@fromorbit.com>]

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