[PATCH 29/34] xfs: convert l_last_sync_lsn to an atomic variable

Dave Chinner
2010-12-21T07:28:42Z

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>
---
fs/xfs/xfs_log.c | 55 +++++++++++++++++++++-------------------------
fs/xfs/xfs_log_priv.h | 9 ++++++-
fs/xfs/xfs_log_recover.c | 6 ++--
3 files changed, 36 insertions(+), 34 deletions(-)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 1e2020d..70790eb 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -675,12 +675,8 @@ xfs_log_move_tail(xfs_mount_t *mp,
if (XLOG_FORCED_SHUTDOWN(log))
return;

- if (tail_lsn == 0) {
- /* needed since sync_lsn is 64 bits */
- spin_lock(&log->l_icloglock);
- tail_lsn = log->l_last_sync_lsn;
- spin_unlock(&log->l_icloglock);
- }
+ if (tail_lsn == 0)
+ tail_lsn = atomic64_read(&log->l_last_sync_lsn);

spin_lock(&log->l_grant_lock);

@@ -800,11 +796,9 @@ xlog_assign_tail_lsn(xfs_mount_t *mp)

tail_lsn = xfs_trans_ail_tail(mp->m_ail);
spin_lock(&log->l_grant_lock);
- if (tail_lsn != 0) {
- log->l_tail_lsn = tail_lsn;
- } else {
- tail_lsn = log->l_tail_lsn = log->l_last_sync_lsn;
- }
+ if (!tail_lsn)
+ tail_lsn = atomic64_read(&log->l_last_sync_lsn);
+ log->l_tail_lsn = tail_lsn;
spin_unlock(&log->l_grant_lock);

return tail_lsn;
@@ -1014,9 +1008,9 @@ xlog_alloc_log(xfs_mount_t *mp,
log->l_flags |= XLOG_ACTIVE_RECOVERY;

log->l_prev_block = -1;
- log->l_tail_lsn = xlog_assign_lsn(1, 0);
/* log->l_tail_lsn = 0x100000000LL; cycle = 1; current block = 0 */
- log->l_last_sync_lsn = log->l_tail_lsn;
+ log->l_tail_lsn = xlog_assign_lsn(1, 0);
+ atomic64_set(&log->l_last_sync_lsn, xlog_assign_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);
@@ -1194,6 +1188,7 @@ xlog_grant_push_ail(
int need_bytes)
{
xfs_lsn_t threshold_lsn = 0;
+ xfs_lsn_t last_sync_lsn;
xfs_lsn_t tail_lsn;
int free_blocks;
int free_bytes;
@@ -1228,10 +1223,12 @@ xlog_grant_push_ail(
threshold_block);
/*
* Don't pass in an lsn greater than the lsn of the last
- * log record known to be on disk.
+ * log record known to be on disk. Use a snapshot of the last sync lsn
+ * so that it doesn't change between the compare and the set.
*/
- if (XFS_LSN_CMP(threshold_lsn, log->l_last_sync_lsn) > 0)
- threshold_lsn = log->l_last_sync_lsn;
+ last_sync_lsn = atomic64_read(&log->l_last_sync_lsn);
+ if (XFS_LSN_CMP(threshold_lsn, last_sync_lsn) > 0)
+ threshold_lsn = last_sync_lsn;

/*
* Get the transaction layer to kick the dirty buffers out to
@@ -2194,7 +2191,7 @@ xlog_state_do_callback(
lowest_lsn = xlog_get_lowest_lsn(log);
if (lowest_lsn &&
XFS_LSN_CMP(lowest_lsn,
- be64_to_cpu(iclog->ic_header.h_lsn)) < 0) {
+ be64_to_cpu(iclog->ic_header.h_lsn)) < 0) {
iclog = iclog->ic_next;
continue; /* Leave this iclog for
* another thread */
@@ -2202,23 +2199,21 @@ xlog_state_do_callback(

iclog->ic_state = XLOG_STATE_CALLBACK;

- spin_unlock(&log->l_icloglock);

- /* l_last_sync_lsn field protected by
- * l_grant_lock. Don't worry about iclog's lsn.
- * No one else can be here except us.
+ /*
+ * update the last_sync_lsn before we drop the
+ * icloglock to ensure we are the only one that
+ * can update it.
*/
- spin_lock(&log->l_grant_lock);
- ASSERT(XFS_LSN_CMP(log->l_last_sync_lsn,
- be64_to_cpu(iclog->ic_header.h_lsn)) <= 0);
- log->l_last_sync_lsn =
- be64_to_cpu(iclog->ic_header.h_lsn);
- spin_unlock(&log->l_grant_lock);
+ ASSERT(XFS_LSN_CMP(atomic64_read(&log->l_last_sync_lsn),
+ be64_to_cpu(iclog->ic_header.h_lsn)) <= 0);
+ atomic64_set(&log->l_last_sync_lsn,
+ be64_to_cpu(iclog->ic_header.h_lsn));

- } else {
- spin_unlock(&log->l_icloglock);
+ } else
ioerrors++;
- }
+
+ spin_unlock(&log->l_icloglock);

/*
* Keep processing entries in the callback list until
diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index e2bb276..958f356 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];
diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 1550404..18e1e18 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -937,7 +937,7 @@ xlog_find_tail(
if (found == 2)
log->l_curr_cycle++;
log->l_tail_lsn = be64_to_cpu(rhead->h_tail_lsn);
- log->l_last_sync_lsn = be64_to_cpu(rhead->h_lsn);
+ atomic64_set(&log->l_last_sync_lsn, be64_to_cpu(rhead->h_lsn));
xlog_assign_grant_head(&log->l_grant_reserve_head, log->l_curr_cycle,
BBTOB(log->l_curr_block));
xlog_assign_grant_head(&log->l_grant_write_head, log->l_curr_cycle,
@@ -989,9 +989,9 @@ xlog_find_tail(
log->l_tail_lsn =
xlog_assign_lsn(log->l_curr_cycle,
after_umount_blk);
- log->l_last_sync_lsn =
+ atomic64_set(&log->l_last_sync_lsn,
xlog_assign_lsn(log->l_curr_cycle,
- after_umount_blk);
+ after_umount_blk));
*tail_blk = after_umount_blk;