Re: [PATCH v3] xfs: don't hold buffer locks across sync transaction commit in xfs_sync_sb_buf

[Darrick J. Wong]

[add xfsprogs maintainer to cc]

On Tue, Sep 15, 2026 at 11:18:47AM +0800, Zhou, Yun wrote:
> 
> 
> On 9/12/26 01:42, Darrick J. Wong wrote:
> > CAUTION: This email comes from a non Wind River email account!
> > Do not click links or open attachments unless you recognize the sender and know the content is safe.
> > 
> > On Fri, Jul 31, 2026 at 11:10:10AM +0800, Yun Zhou wrote:
> > > xfs_sync_sb_buf() holds sb/rtsb buffer locks across a synchronous
> > > xfs_trans_commit(), which flushes the CIL push workqueue internally.
> > > If shutdown occurs during the CIL push, xfs_buf_item_unpin() needs to
> > > lock these buffers to fail them, causing a deadlock:
> > > 
> > >    setlabel:          holds buf lock -> flush_workqueue(xfs-cil)
> > >    CIL push worker:   xfs_buf_item_unpin -> xfs_buf_lock(same buf)
> > > 
> > > Remove the xfs_trans_bhold() calls so that commit releases the buffer
> > > locks normally.  After the sync commit, re-acquire the buffers via
> > > mp->m_sb_bp / mp->m_rtsb_bp for the on-disk writeback.
> > > 
> > > Fixes: f7664b31975b ("xfs: implement online get/set fs label")
> > > Reported-by: syzbot+837bcd54843dd6262f2f@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Closes: https://syzkaller.appspot.com/bug?extid=837bcd54843dd6262f2f
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
> > > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > > ---
> > > v3:
> > >   - Drop xfs_buf_hold/xfs_buf_relse, use lock/unlock directly (Christoph).
> > > 
> > > v2:
> > >   - Remove the bp variable and pass xfs_trans_getsb(tp) directly to
> > >     xfs_log_rtsb() to fix compilation warnings when CONFIG_XFS_RT=n.
> > >   - Convert xfs_log_rtsb() stub from macro to inline function to avoid
> > >     the need for (void) casting (Christoph).
> > > ---
> > >   fs/xfs/libxfs/xfs_rtgroup.h |  6 +++++-
> > >   fs/xfs/libxfs/xfs_sb.c      | 37 +++++++++++++++++--------------------
> > >   2 files changed, 22 insertions(+), 21 deletions(-)
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_rtgroup.h b/fs/xfs/libxfs/xfs_rtgroup.h
> > > index c0b9f9f2c413..fca2eb74908c 100644
> > > --- a/fs/xfs/libxfs/xfs_rtgroup.h
> > > +++ b/fs/xfs/libxfs/xfs_rtgroup.h
> > > @@ -359,7 +359,11 @@ static inline int xfs_initialize_rtgroups(struct xfs_mount *mp,
> > >   # define xfs_rtgroup_unlock(rtg, gf)         ((void)0)
> > >   # define xfs_rtgroup_trans_join(tp, rtg, gf) ((void)0)
> > >   # define xfs_update_rtsb(bp, sb_bp)  ((void)0)
> > > -# define xfs_log_rtsb(tp, sb_bp)     (NULL)
> > > +static inline struct xfs_buf *xfs_log_rtsb(struct xfs_trans *tp,
> > > +             const struct xfs_buf *sb_bp)
> > > +{
> > > +     return NULL;
> > > +}
> > >   # define xfs_rtgroup_get_geometry(rtg, rgeo) (-EOPNOTSUPP)
> > >   #endif /* CONFIG_XFS_RT */
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> > > index 47322adb7690..2664728f2d8f 100644
> > > --- a/fs/xfs/libxfs/xfs_sb.c
> > > +++ b/fs/xfs/libxfs/xfs_sb.c
> > > @@ -1470,36 +1470,33 @@ xfs_sync_sb_buf(
> > >        bool                    update_rtsb)
> > >   {
> > >        struct xfs_trans        *tp;
> > > -     struct xfs_buf          *bp;
> > > -     struct xfs_buf          *rtsb_bp = NULL;
> > >        int                     error;
> > > 
> > >        error = xfs_trans_alloc(mp, &M_RES(mp)->tr_sb, 0, 0, 0, &tp);
> > >        if (error)
> > >                return error;
> > > 
> > > -     bp = xfs_trans_getsb(tp);
> > >        xfs_log_sb(tp);
> > > -     xfs_trans_bhold(tp, bp);
> > > -     if (update_rtsb) {
> > > -             rtsb_bp = xfs_log_rtsb(tp, bp);
> > > -             if (rtsb_bp)
> > > -                     xfs_trans_bhold(tp, rtsb_bp);
> > > -     }
> > > +     if (update_rtsb)
> > > +             xfs_log_rtsb(tp, xfs_trans_getsb(tp));
> > >        xfs_trans_set_sync(tp);
> > >        error = xfs_trans_commit(tp);
> > >        if (error)
> > > -             goto out;
> > > -     /*
> > > -      * write out the sb buffer to get the changes to disk
> > > -      */
> > > -     error = xfs_bwrite(bp);
> > > -     if (!error && rtsb_bp)
> > > -             error = xfs_bwrite(rtsb_bp);
> > > -out:
> > > -     if (rtsb_bp)
> > > -             xfs_buf_relse(rtsb_bp);
> > > -     xfs_buf_relse(bp);
> > > +             return error;
> > > +
> > > +     /* Re-acquire and write the sb and rtsb to disk. */
> > > +     xfs_buf_lock(mp->m_sb_bp);
> > 
> > FYI, this causes porting problems to xfsprogs' libxfs because userspace
> > doesn't have m_sb_bp or m_rtsb_bp pointers in struct xfs_mount.  It's
> > not a big deal to add them, but if anyone ever wants to call
> > xfs_sync_sb_buf then we're going to have to figure out when to populate
> > those pointers.
> > 
> 
> You're right — the problem is referencing mp->m_sb_bp / mp->m_rtsb_bp
> directly in libxfs. I can avoid that by going back to acquiring the
> buffer early via xfs_trans_getsb() / xfs_log_rtsb() (as the original
> code did), instead of touching the mount pointers.
> 
> To keep this safe for a userspace caller that may not hold a
> mount-level reference to the sb buffer, we can take an explicit
> xfs_buf_hold() after getsb so the buffer stays alive across the
> commit. Unlike the original xfs_trans_bhold(), xfs_buf_hold() only
> takes a reference and does not keep the buffer locked across the
> commit, so it doesn't reintroduce the deadlock.
> 
> Alternatively, since xfs_ioc_setlabel() is currently the only caller
> and it's kernel-only, we could just move xfs_sync_sb_buf() out of
> libxfs into xfs_ioctl.c.

Probably safer to do that, since there's no other user of that function,
and none in userspace.

> Since this is already in for-next, would you prefer an incremental follow-up
> patch, or should I resend as v4?

It's already been merged, so whatever fix we come up with has to be a
new patch.

--D

> Thanks,
> Yun
> 
>