reduce l_icloglock roundtrips

All but one caller of xlog_state_want_sync drop and re-acquire
l_icloglock around the call to it, just so that xlog_state_want_sync can
acquire and drop it.

Move all lock operation out of l_icloglock and assert that the lock is
held when it is called.

Note that it would make sense to extende this scheme to
xlog_state_release_iclog, but the locking in there is more complicated
and we'd like to keep the atomic_dec_and_lock optmization for those
callers not having l_icloglock yet.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>

fs/xfs/xfs_log.c

@@ -729,8 +729,8 @@ xfs_log_unmount_write(xfs_mount_t *mp)
 		spin_lock(&log->l_icloglock);
 		iclog = log->l_iclog;
 		atomic_inc(&iclog->ic_refcnt);
-		spin_unlock(&log->l_icloglock);
 		xlog_state_want_sync(log, iclog);
+		spin_unlock(&log->l_icloglock);
 		error = xlog_state_release_iclog(log, iclog);
 
 		spin_lock(&log->l_icloglock);
@@ -767,9 +767,9 @@ xfs_log_unmount_write(xfs_mount_t *mp)
 		spin_lock(&log->l_icloglock);
 		iclog = log->l_iclog;
 		atomic_inc(&iclog->ic_refcnt);
-		spin_unlock(&log->l_icloglock);
 
 		xlog_state_want_sync(log, iclog);
+		spin_unlock(&log->l_icloglock);
 		error =  xlog_state_release_iclog(log, iclog);
 
 		spin_lock(&log->l_icloglock);
@@ -1984,7 +1984,9 @@ xlog_write(xfs_mount_t *	mp,
 		if (iclog->ic_size - log_offset <= sizeof(xlog_op_header_t)) {
 		    xlog_state_finish_copy(log, iclog, record_cnt, data_cnt);
 		    record_cnt = data_cnt = 0;
+		    spin_lock(&log->l_icloglock);
 		    xlog_state_want_sync(log, iclog);
+		    spin_unlock(&log->l_icloglock);
 		    if (commit_iclog) {
 			ASSERT(flags & XLOG_COMMIT_TRANS);
 			*commit_iclog = iclog;
@@ -3193,18 +3195,15 @@ xlog_state_sync(xlog_t	  *log,
 STATIC void
 xlog_state_want_sync(xlog_t *log, xlog_in_core_t *iclog)
 {
-	spin_lock(&log->l_icloglock);
+	ASSERT(spin_is_locked(&log->l_icloglock));
 
 	if (iclog->ic_state == XLOG_STATE_ACTIVE) {
 		xlog_state_switch_iclogs(log, iclog, 0);
 	} else {
 		ASSERT(iclog->ic_state &
 			(XLOG_STATE_WANT_SYNC|XLOG_STATE_IOERROR));
 	}
-
-	spin_unlock(&log->l_icloglock);
-}	/* xlog_state_want_sync */
-
+}
 
 
 /*****************************************************************************