xfs: convert l_tail_lsn to an atomic variable.

log->l_tail_lsn is currently protected by the log grant lock. The
lock is only needed for serialising readers against writers, so we
don't really need the lock if we make the l_tail_lsn variable an
atomic. Converting the l_tail_lsn variable to an atomic64_t means we
can start to peel back the grant lock from various operations.

Also, provide functions to safely crack an atomic LSN variable into
it's component pieces and to recombined the components into an
atomic variable. Use them where appropriate.

This also removes the need for explicitly holding a spinlock to read
the l_tail_lsn on 32 bit platforms.

Signed-off-by: Dave Chinner <dchinner@redhat.com>

1 files changed, 25 insertions, 31 deletions

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 70790eb..d118bf8 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -678,15 +678,11 @@ xfs_log_move_tail(xfs_mount_t	*mp,
 	if (tail_lsn == 0)
 		tail_lsn = atomic64_read(&log->l_last_sync_lsn);
 
-	spin_lock(&log->l_grant_lock);
-
-	/* Also an invalid lsn.  1 implies that we aren't passing in a valid
-	 * tail_lsn.
-	 */
-	if (tail_lsn != 1) {
-		log->l_tail_lsn = tail_lsn;
-	}
+	/* tail_lsn == 1 implies that we weren't passed a valid value.  */
+	if (tail_lsn != 1)
+		atomic64_set(&log->l_tail_lsn, tail_lsn);
 
+	spin_lock(&log->l_grant_lock);
 	if (!list_empty(&log->l_writeq)) {
 #ifdef DEBUG
 		if (log->l_flags & XLOG_ACTIVE_RECOVERY)
@@ -789,21 +785,19 @@ xfs_log_need_covered(xfs_mount_t *mp)
  * We may be holding the log iclog lock upon entering this routine.
  */
 xfs_lsn_t
-xlog_assign_tail_lsn(xfs_mount_t *mp)
+xlog_assign_tail_lsn(
+	struct xfs_mount	*mp)
 {
-	xfs_lsn_t tail_lsn;
-	xlog_t	  *log = mp->m_log;
+	xfs_lsn_t		tail_lsn;
+	struct log		*log = mp->m_log;
 
 	tail_lsn = xfs_trans_ail_tail(mp->m_ail);
-	spin_lock(&log->l_grant_lock);
 	if (!tail_lsn)
 		tail_lsn = atomic64_read(&log->l_last_sync_lsn);
-	log->l_tail_lsn = tail_lsn;
-	spin_unlock(&log->l_grant_lock);
 
+	atomic64_set(&log->l_tail_lsn, tail_lsn);
 	return tail_lsn;
-}	/* xlog_assign_tail_lsn */
-
+}
 
 /*
  * Return the space in the log between the tail and the head.  The head
@@ -831,8 +825,8 @@ xlog_space_left(
 	int		head_bytes;
 
 	xlog_crack_grant_head(head, &head_cycle, &head_bytes);
-	tail_bytes = BBTOB(BLOCK_LSN(log->l_tail_lsn));
-	tail_cycle = CYCLE_LSN(log->l_tail_lsn);
+	xlog_crack_atomic_lsn(&log->l_tail_lsn, &tail_cycle, &tail_bytes);
+	tail_bytes = BBTOB(tail_bytes);
 	if (tail_cycle == head_cycle && head_bytes >= tail_bytes)
 		free_bytes = log->l_logsize - (head_bytes - tail_bytes);
 	else if (tail_cycle + 1 < head_cycle)
@@ -1009,8 +1003,8 @@ xlog_alloc_log(xfs_mount_t	*mp,
 
 	log->l_prev_block  = -1;
 	/* log->l_tail_lsn = 0x100000000LL; cycle = 1; current block = 0 */
-	log->l_tail_lsn	   = xlog_assign_lsn(1, 0);
-	atomic64_set(&log->l_last_sync_lsn, xlog_assign_lsn(1, 0));
+	xlog_assign_atomic_lsn(&log->l_tail_lsn, 1, 0);
+	xlog_assign_atomic_lsn(&log->l_last_sync_lsn, 1, 0);
 	log->l_curr_cycle  = 1;	    /* 0 is bad since this is initial value */
 	xlog_assign_grant_head(&log->l_grant_reserve_head, 1, 0);
 	xlog_assign_grant_head(&log->l_grant_write_head, 1, 0);
@@ -1189,7 +1183,6 @@ xlog_grant_push_ail(
 {
 	xfs_lsn_t	threshold_lsn = 0;
 	xfs_lsn_t	last_sync_lsn;
-	xfs_lsn_t	tail_lsn;
 	int		free_blocks;
 	int		free_bytes;
 	int		threshold_block;
@@ -1198,7 +1191,6 @@ xlog_grant_push_ail(
 
 	ASSERT(BTOBB(need_bytes) < log->l_logBBsize);
 
-	tail_lsn = log->l_tail_lsn;
 	free_bytes = xlog_space_left(log, &log->l_grant_reserve_head);
 	free_blocks = BTOBBT(free_bytes);
 
@@ -1213,8 +1205,9 @@ xlog_grant_push_ail(
 	if (free_blocks >= free_threshold)
 		return;
 
-	threshold_block = BLOCK_LSN(tail_lsn) + free_threshold;
-	threshold_cycle = CYCLE_LSN(tail_lsn);
+	xlog_crack_atomic_lsn(&log->l_tail_lsn, &threshold_cycle,
+						&threshold_block);
+	threshold_block += free_threshold;
 	if (threshold_block >= log->l_logBBsize) {
 		threshold_block -= log->l_logBBsize;
 		threshold_cycle += 1;
@@ -2828,11 +2821,11 @@ xlog_state_release_iclog(
 
 	if (iclog->ic_state == XLOG_STATE_WANT_SYNC) {
 		/* update tail before writing to iclog */
-		xlog_assign_tail_lsn(log->l_mp);
+		xfs_lsn_t tail_lsn = xlog_assign_tail_lsn(log->l_mp);
 		sync++;
 		iclog->ic_state = XLOG_STATE_SYNCING;
-		iclog->ic_header.h_tail_lsn = cpu_to_be64(log->l_tail_lsn);
-		xlog_verify_tail_lsn(log, iclog, log->l_tail_lsn);
+		iclog->ic_header.h_tail_lsn = cpu_to_be64(tail_lsn);
+		xlog_verify_tail_lsn(log, iclog, tail_lsn);
 		/* cycle incremented when incrementing curr_block */
 	}
 	spin_unlock(&log->l_icloglock);
@@ -3435,7 +3428,7 @@ STATIC void
 xlog_verify_grant_tail(
 	struct log	*log)
 {
-	xfs_lsn_t	tail_lsn = log->l_tail_lsn;
+	int		tail_cycle, tail_blocks;
 	int		cycle, space;
 
 	/*
@@ -3445,9 +3438,10 @@ xlog_verify_grant_tail(
 	 * check the byte count.
 	 */
 	xlog_crack_grant_head(&log->l_grant_write_head, &cycle, &space);
-	if (CYCLE_LSN(tail_lsn) != cycle) {
-		ASSERT(cycle - 1 == CYCLE_LSN(tail_lsn));
-		ASSERT(space <= BBTOB(BLOCK_LSN(tail_lsn)));
+	xlog_crack_atomic_lsn(&log->l_tail_lsn, &tail_cycle, &tail_blocks);
+	if (tail_cycle != cycle) {
+		ASSERT(cycle - 1 == tail_cycle);
+		ASSERT(space <= BBTOB(tail_blocks));
 	}
 }