Re: [PATCH v2] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim

[Waiman Long]

On 6/4/20 7:13 PM, Dave Chinner wrote:
> On Thu, Jun 04, 2020 at 05:01:30PM -0400, Waiman Long wrote:
> > ---
> >   fs/xfs/xfs_log.c   | 3 ++-
> >   fs/xfs/xfs_trans.c | 8 +++++++-
> >   2 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> > index 00fda2e8e738..d273d4e74ef8 100644
> > --- a/fs/xfs/xfs_log.c
> > +++ b/fs/xfs/xfs_log.c
> > @@ -433,7 +433,8 @@ xfs_log_reserve(
> >   	XFS_STATS_INC(mp, xs_try_logspace);
> >   
> >   	ASSERT(*ticp == NULL);
> > -	tic = xlog_ticket_alloc(log, unit_bytes, cnt, client, permanent, 0);
> > +	tic = xlog_ticket_alloc(log, unit_bytes, cnt, client, permanent,
> > +			mp->m_super->s_writers.frozen ? KM_NOLOCKDEP : 0);
> >   	*ticp = tic;
> Hi Waiman,
>
> As I originally stated when you posted this the first time 6 months
> ago: we are not going to spread this sort of conditional gunk though
> the XFS codebase just to shut up lockdep false positives.
>
> I pointed you at the way to conditionally turn of lockdep for
> operations where we are doing transactions when the filesystem has
> already frozen the transaction subsystem. That is:
>
> >   
> >   	xlog_grant_push_ail(log, tic->t_cnt ? tic->t_unit_res * tic->t_cnt
> > diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> > index 3c94e5ff4316..3a9f394a0f02 100644
> > --- a/fs/xfs/xfs_trans.c
> > +++ b/fs/xfs/xfs_trans.c
> > @@ -261,8 +261,14 @@ xfs_trans_alloc(
> >   	 * Allocate the handle before we do our freeze accounting and setting up
> >   	 * GFP_NOFS allocation context so that we avoid lockdep false positives
> >   	 * by doing GFP_KERNEL allocations inside sb_start_intwrite().
> > +	 *
> > +	 * To prevent false positive lockdep warning of circular locking
> > +	 * dependency between sb_internal and fs_reclaim, disable the
> > +	 * acquisition of the fs_reclaim pseudo-lock when the superblock
> > +	 * has been frozen or in the process of being frozen.
> >   	 */
> > -	tp = kmem_zone_zalloc(xfs_trans_zone, 0);
> > +	tp = kmem_zone_zalloc(xfs_trans_zone,
> > +		mp->m_super->s_writers.frozen ? KM_NOLOCKDEP : 0);
> >   	if (!(flags & XFS_TRANS_NO_WRITECOUNT))
> We only should be setting KM_NOLOCKDEP when XFS_TRANS_NO_WRITECOUNT
> is set.  That's the flag that transactions set when they run in a
> fully frozen context to avoid deadlocking with the freeze in
> progress, and that's the only case where we should be turning off
> lockdep.
>
> And, as I also mentioned, this should be done via a process flag -
> PF_MEMALLOC_NOLOCKDEP - so that it is automatically inherited by
> all subsequent memory allocations done in this path. That way we
> only need this wrapping code in xfs_trans_alloc():
>
> 	if (flags & XFS_TRANS_NO_WRITECOUNT)
> 		memalloc_nolockdep_save()
>
> 	.....
>
> 	if (flags & XFS_TRANS_NO_WRITECOUNT)
> 		memalloc_nolockdep_restore()
>
> and nothing else needs to change.
>
> Cheers,
>
> Dave.

Thanks for the reminder, I will look into that.

Cheers,
Longman