xfs: convert xfs_ialloc_next_ag() to an atomic

This is currently a spinlock lock protected rotor which can be
implemented with a single atomic operation. Change it to be more
efficient and get rid of the m_agirotor_lock. Noticed while
converting the inode allocation AG selection loop to active perag
references.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/libxfs/xfs_ialloc.c

@@ -1576,21 +1576,6 @@ xfs_dialloc_roll(
 	return error;
 }
 
-static xfs_agnumber_t
-xfs_ialloc_next_ag(
-	xfs_mount_t	*mp)
-{
-	xfs_agnumber_t	agno;
-
-	spin_lock(&mp->m_agirotor_lock);
-	agno = mp->m_agirotor;
-	if (++mp->m_agirotor >= mp->m_maxagi)
-		mp->m_agirotor = 0;
-	spin_unlock(&mp->m_agirotor_lock);
-
-	return agno;
-}
-
 static bool
 xfs_dialloc_good_ag(
 	struct xfs_perag	*pag,
@@ -1748,7 +1733,7 @@ xfs_dialloc(
 	 * an AG has enough space for file creation.
 	 */
 	if (S_ISDIR(mode))
-		start_agno = xfs_ialloc_next_ag(mp);
+		start_agno = atomic_inc_return(&mp->m_agirotor) % mp->m_maxagi;
 	else {
 		start_agno = XFS_INO_TO_AGNO(mp, parent);
 		if (start_agno >= mp->m_maxagi)

fs/xfs/libxfs/xfs_sb.c

@@ -909,7 +909,8 @@ xfs_sb_mount_common(
 	struct xfs_mount	*mp,
 	struct xfs_sb		*sbp)
 {
-	mp->m_agfrotor = mp->m_agirotor = 0;
+	mp->m_agfrotor = 0;
+	atomic_set(&mp->m_agirotor, 0);
 	mp->m_maxagi = mp->m_sb.sb_agcount;
 	mp->m_blkbit_log = sbp->sb_blocklog + XFS_NBBYLOG;
 	mp->m_blkbb_log = sbp->sb_blocklog - BBSHIFT;

fs/xfs/xfs_mount.h

@@ -210,8 +210,7 @@ typedef struct xfs_mount {
 	struct xfs_error_cfg	m_error_cfg[XFS_ERR_CLASS_MAX][XFS_ERR_ERRNO_MAX];
 	struct xstats		m_stats;	/* per-fs stats */
 	xfs_agnumber_t		m_agfrotor;	/* last ag where space found */
-	xfs_agnumber_t		m_agirotor;	/* last ag dir inode alloced */
-	spinlock_t		m_agirotor_lock;/* .. and lock protecting it */
+	atomic_t		m_agirotor;	/* last ag dir inode alloced */
 
 	/* Memory shrinker to throttle and reprioritize inodegc */
 	struct shrinker		m_inodegc_shrinker;

fs/xfs/xfs_super.c

@@ -1922,7 +1922,6 @@ static int xfs_init_fs_context(
 		return -ENOMEM;
 
 	spin_lock_init(&mp->m_sb_lock);
-	spin_lock_init(&mp->m_agirotor_lock);
 	INIT_RADIX_TREE(&mp->m_perag_tree, GFP_ATOMIC);
 	spin_lock_init(&mp->m_perag_lock);
 	mutex_init(&mp->m_growlock);