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

Waiman Long
2020-06-22T17:56:16Z

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