Re: [PATCH v2] xfs: don't hold buffer locks across sync transaction commit in xfs_sync_sb_buf

[Christoph Hellwig]

On Wed, Jul 22, 2026 at 09:38:44PM +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
> Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
> ---
> Changes in 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      | 39 ++++++++++++++++++-------------------
>  2 files changed, 24 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..929677ad95b4 100644
> --- a/fs/xfs/libxfs/xfs_sb.c
> +++ b/fs/xfs/libxfs/xfs_sb.c
> @@ -1470,36 +1470,35 @@ xfs_sync_sb_buf(
>  	bool			update_rtsb)
>  {
>  	struct xfs_trans	*tp;
>  	int			error;
>  
>  	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_sb, 0, 0, 0, &tp);
>  	if (error)
>  		return error;
>  
>  	xfs_log_sb(tp);
> +	if (update_rtsb)
> +		xfs_log_rtsb(tp, xfs_trans_getsb(tp));
>  	xfs_trans_set_sync(tp);

No new here, but I don't think the transaction reservation is correct
here.  If we're writing both the sb and rtsb buffers, we need a log
reservation for two buffers, not just for one.  Separate patch,
though.

>
>  	error = xfs_trans_commit(tp);
>  	if (error)
> +		return error;
> +
> +	/* Re-acquire and write the sb and rtsb to disk. */
> +	xfs_buf_lock(mp->m_sb_bp);
> +	xfs_buf_hold(mp->m_sb_bp);
> +	error = xfs_bwrite(mp->m_sb_bp);
> +	xfs_buf_relse(mp->m_sb_bp);
> +	if (error)
> +		return error;
> +
> +	if (update_rtsb && mp->m_rtsb_bp) {
> +		xfs_buf_lock(mp->m_rtsb_bp);
> +		xfs_buf_hold(mp->m_rtsb_bp);
> +		error = xfs_bwrite(mp->m_rtsb_bp);
> +		xfs_buf_relse(mp->m_rtsb_bp);
> +	}

I don't think we need an extra hold here for both buffers, just
a lock/unlock, or am I missing something?

Anyway, the fix itself looks good, so:

Reviewed-by: Christoph Hellwig <hch@xxxxxx>