lkml.org 
[lkml]   [2020]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim
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.
--
Dave Chinner
david@fromorbit.com

\
 
 \ /
  Last update: 2020-06-05 01:13    [W:0.064 / U:0.180 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site