Messages in this thread | | | Subject | Re: [PATCH v4] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim | From | Waiman Long <> | Date | Mon, 22 Jun 2020 13:56:04 -0400 |
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On 6/18/20 7:04 PM, Dave Chinner wrote: > 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...
OK, how about simplifying the comment to as follows:
/* * Disable fs reclaim in memory allocation for fs freeze to avoid * causing a possible circular locking dependency lockdep splat * involving fs reclaim. */
Does that look good enough for you?
Cheers, Longman
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