xfs: convert l_last_sync_lsn to an atomic variable

log->l_last_sync_lsn is updated in only one critical spot - log
buffer Io completion - and is protected by the grant lock here. This
requires the grant lock to be taken for every log buffer IO
completion. Converting the l_last_sync_lsn variable to an atomic64_t
means that we do not need to take the grant lock in log buffer IO
completion to update it.

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

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 8 insertions, 1 deletions

diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index e2bb276eb2a7..958f356df10e 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -507,7 +507,6 @@ typedef struct log {
         spinlock_t              l_icloglock;    /* grab to change iclog state */
         xfs_lsn_t               l_tail_lsn;     /* lsn of 1st LR with unflushed
                                                  * buffers */
-        xfs_lsn_t               l_last_sync_lsn;/* lsn of last LR on disk */
         int                     l_curr_cycle;   /* Cycle number of log writes */
         int                     l_prev_cycle;   /* Cycle number before last
                                                  * block increment */
@@ -521,6 +520,14 @@ typedef struct log {
         int64_t                 l_grant_reserve_head;
         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.
+         */
+        /* lsn of last LR on disk */
+        atomic64_t              l_last_sync_lsn ____cacheline_aligned_in_smp;
+
         /* The following field are used for debugging; need to hold icloglock */
 #ifdef DEBUG
         char                    *l_iclog_bak[XLOG_MAX_ICLOGS];