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, 31 insertions, 6 deletions

diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index 958f356df10e..d34af1c21ed2 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -53,7 +53,6 @@ struct xfs_mount;
 	BTOBB(XLOG_MAX_ICLOGS << (xfs_sb_version_haslogv2(&log->l_mp->m_sb) ? \
 	 XLOG_MAX_RECORD_BSHIFT : XLOG_BIG_RECORD_BSHIFT))
 
-
 static inline xfs_lsn_t xlog_assign_lsn(uint cycle, uint block)
 {
 	return ((xfs_lsn_t)cycle << 32) | block;
@@ -505,8 +504,6 @@ typedef struct log {
 						 * log entries" */
 	xlog_in_core_t		*l_iclog;       /* head log queue	*/
 	spinlock_t		l_icloglock;    /* grab to change iclog state */
-	xfs_lsn_t		l_tail_lsn;     /* lsn of 1st LR with unflushed
-						 * buffers */
 	int			l_curr_cycle;   /* Cycle number of log writes */
 	int			l_prev_cycle;   /* Cycle number before last
 						 * block increment */
@@ -521,12 +518,15 @@ typedef struct log {
 	int64_t			l_grant_write_head;
 
 	/*
-	 * l_last_sync_lsn is an atomic so it can be set and read without
-	 * needing to hold specific locks. To avoid operations contending with
-	 * other hot objects, place it on a separate cacheline.
+	 * l_last_sync_lsn and l_tail_lsn are atomics so they can be set and
+	 * read without needing to hold specific locks. To avoid operations
+	 * contending with other hot objects, place each of them on a separate
+	 * cacheline.
 	 */
 	/* lsn of last LR on disk */
 	atomic64_t		l_last_sync_lsn ____cacheline_aligned_in_smp;
+	/* lsn of 1st LR with unflushed * buffers */
+	atomic64_t		l_tail_lsn ____cacheline_aligned_in_smp;
 
 	/* The following field are used for debugging; need to hold icloglock */
 #ifdef DEBUG
@@ -566,6 +566,31 @@ int	xlog_write(struct log *log, struct xfs_log_vec *log_vector,
 				xlog_in_core_t **commit_iclog, uint flags);
 
 /*
+ * When we crack an atomic LSN, we sample it first so that the value will not
+ * change while we are cracking it into the component values. This means we
+ * will always get consistent component values to work from. This should always
+ * be used to smaple and crack LSNs taht are stored and updated in atomic
+ * variables.
+ */
+static inline void
+xlog_crack_atomic_lsn(atomic64_t *lsn, uint *cycle, uint *block)
+{
+	xfs_lsn_t val = atomic64_read(lsn);
+
+	*cycle = CYCLE_LSN(val);
+	*block = BLOCK_LSN(val);
+}
+
+/*
+ * Calculate and assign a value to an atomic LSN variable from component pieces.
+ */
+static inline void
+xlog_assign_atomic_lsn(atomic64_t *lsn, uint cycle, uint block)
+{
+	atomic64_set(lsn, xlog_assign_lsn(cycle, block));
+}
+
+/*
  * When we crack the grrant head, we sample it first so that the value will not
  * change while we are cracking it into the component values. This means we
  * will always get consistent component values to work from.