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

To: xfs@xxxxxxxxxxx
Subject: [PATCH 29/34] xfs: convert l_last_sync_lsn to an atomic variable
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 21 Dec 2010 18:29:25 +1100
In-reply-to: <1292916570-25015-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1292916570-25015-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

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@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
---
 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;
 
                        /*
-- 
1.7.2.3

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