[XFS] Use atomics for iclog reference counting

Now that we update the log tail LSN less frequently on transaction
completion, we pass the contention straight to the global log state lock
(l_iclog_lock) during transaction completion.

We currently have to take this lock to decrement the iclog reference
count. there is a reference count on each iclog, so we need to take þhe
global lock for all refcount changes.

When large numbers of processes are all doing small trnasctions, the iclog
reference counts will be quite high, and the state change that absolutely
requires the l_iclog_lock is the except rather than the norm.

Change the reference counting on the iclogs to use atomic_inc/dec so that
we can use atomic_dec_and_lock during transaction completion and avoid the
need for grabbing the l_iclog_lock for every reference count decrement
except the one that matters - the last.

SGI-PV: 975671
SGI-Modid: xfs-linux-melb:xfs-kern:30505a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>

fs/xfs/xfs_log.c

@@ -675,7 +675,7 @@ xfs_log_unmount_write(xfs_mount_t *mp)
 
 		spin_lock(&log->l_icloglock);
 		iclog = log->l_iclog;
-		iclog->ic_refcnt++;
+		atomic_inc(&iclog->ic_refcnt);
 		spin_unlock(&log->l_icloglock);
 		xlog_state_want_sync(log, iclog);
 		(void) xlog_state_release_iclog(log, iclog);
@@ -713,7 +713,7 @@ xfs_log_unmount_write(xfs_mount_t *mp)
 		 */
 		spin_lock(&log->l_icloglock);
 		iclog = log->l_iclog;
-		iclog->ic_refcnt++;
+		atomic_inc(&iclog->ic_refcnt);
 		spin_unlock(&log->l_icloglock);
 
 		xlog_state_want_sync(log, iclog);
@@ -1405,7 +1405,7 @@ xlog_sync(xlog_t		*log,
 	int		v2 = xfs_sb_version_haslogv2(&log->l_mp->m_sb);
 
 	XFS_STATS_INC(xs_log_writes);
-	ASSERT(iclog->ic_refcnt == 0);
+	ASSERT(atomic_read(&iclog->ic_refcnt) == 0);
 
 	/* Add for LR header */
 	count_init = log->l_iclog_hsize + iclog->ic_offset;
@@ -2309,7 +2309,7 @@ xlog_state_done_syncing(
 
 	ASSERT(iclog->ic_state == XLOG_STATE_SYNCING ||
 	       iclog->ic_state == XLOG_STATE_IOERROR);
-	ASSERT(iclog->ic_refcnt == 0);
+	ASSERT(atomic_read(&iclog->ic_refcnt) == 0);
 	ASSERT(iclog->ic_bwritecnt == 1 || iclog->ic_bwritecnt == 2);
 
 
@@ -2391,7 +2391,7 @@ xlog_state_get_iclog_space(xlog_t	  *log,
 	ASSERT(iclog->ic_state == XLOG_STATE_ACTIVE);
 	head = &iclog->ic_header;
 
-	iclog->ic_refcnt++;			/* prevents sync */
+	atomic_inc(&iclog->ic_refcnt);	/* prevents sync */
 	log_offset = iclog->ic_offset;
 
 	/* On the 1st write to an iclog, figure out lsn.  This works
@@ -2423,12 +2423,12 @@ xlog_state_get_iclog_space(xlog_t	  *log,
 		xlog_state_switch_iclogs(log, iclog, iclog->ic_size);
 
 		/* If I'm the only one writing to this iclog, sync it to disk */
-		if (iclog->ic_refcnt == 1) {
+		if (atomic_read(&iclog->ic_refcnt) == 1) {
 			spin_unlock(&log->l_icloglock);
 			if ((error = xlog_state_release_iclog(log, iclog)))
 				return error;
 		} else {
-			iclog->ic_refcnt--;
+			atomic_dec(&iclog->ic_refcnt);
 			spin_unlock(&log->l_icloglock);
 		}
 		goto restart;
@@ -2819,18 +2819,21 @@ xlog_state_release_iclog(
 {
 	int		sync = 0;	/* do we sync? */
 
-	spin_lock(&log->l_icloglock);
+	if (iclog->ic_state & XLOG_STATE_IOERROR)
+		return XFS_ERROR(EIO);
+
+	ASSERT(atomic_read(&iclog->ic_refcnt) > 0);
+	if (!atomic_dec_and_lock(&iclog->ic_refcnt, &log->l_icloglock))
+		return 0;
+
 	if (iclog->ic_state & XLOG_STATE_IOERROR) {
 		spin_unlock(&log->l_icloglock);
 		return XFS_ERROR(EIO);
 	}
-
-	ASSERT(iclog->ic_refcnt > 0);
 	ASSERT(iclog->ic_state == XLOG_STATE_ACTIVE ||
 	       iclog->ic_state == XLOG_STATE_WANT_SYNC);
 
-	if (--iclog->ic_refcnt == 0 &&
-	    iclog->ic_state == XLOG_STATE_WANT_SYNC) {
+	if (iclog->ic_state == XLOG_STATE_WANT_SYNC) {
 		/* update tail before writing to iclog */
 		xlog_assign_tail_lsn(log->l_mp);
 		sync++;
@@ -2950,22 +2953,23 @@ xlog_state_sync_all(xlog_t *log, uint flags, int *log_flushed)
 		 * previous iclog and go to sleep.
 		 */
 		if (iclog->ic_state == XLOG_STATE_DIRTY ||
-		    (iclog->ic_refcnt == 0 && iclog->ic_offset == 0)) {
+		    (atomic_read(&iclog->ic_refcnt) == 0
+		     && iclog->ic_offset == 0)) {
 			iclog = iclog->ic_prev;
 			if (iclog->ic_state == XLOG_STATE_ACTIVE ||
 			    iclog->ic_state == XLOG_STATE_DIRTY)
 				goto no_sleep;
 			else
 				goto maybe_sleep;
 		} else {
-			if (iclog->ic_refcnt == 0) {
+			if (atomic_read(&iclog->ic_refcnt) == 0) {
 				/* We are the only one with access to this
 				 * iclog.  Flush it out now.  There should
 				 * be a roundoff of zero to show that someone
 				 * has already taken care of the roundoff from
 				 * the previous sync.
 				 */
-				iclog->ic_refcnt++;
+				atomic_inc(&iclog->ic_refcnt);
 				lsn = be64_to_cpu(iclog->ic_header.h_lsn);
 				xlog_state_switch_iclogs(log, iclog, 0);
 				spin_unlock(&log->l_icloglock);
@@ -3097,7 +3101,7 @@ xlog_state_sync(xlog_t	  *log,
 			already_slept = 1;
 			goto try_again;
 		} else {
-			iclog->ic_refcnt++;
+			atomic_inc(&iclog->ic_refcnt);
 			xlog_state_switch_iclogs(log, iclog, 0);
 			spin_unlock(&log->l_icloglock);
 			if (xlog_state_release_iclog(log, iclog))

fs/xfs/xfs_log_priv.h

@@ -339,7 +339,7 @@ typedef struct xlog_iclog_fields {
 #endif
 	int			ic_size;
 	int			ic_offset;
-	int			ic_refcnt;
+	atomic_t		ic_refcnt;
 	int			ic_bwritecnt;
 	ushort_t		ic_state;
 	char			*ic_datap;	/* pointer to iclog data */