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[PATCH 07/16] xfs: convert inode cache lookups to use RCU locking

To: xfs@xxxxxxxxxxx
Subject: [PATCH 07/16] xfs: convert inode cache lookups to use RCU locking
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 8 Nov 2010 19:55:10 +1100
In-reply-to: <1289206519-18377-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1289206519-18377-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

With delayed logging greatly increasing the sustained parallelism of inode
operations, the inode cache locking is showing significant read vs write
contention when inode reclaim runs at the same time as lookups. There is
also a lot more write lock acquistions than there are read locks (4:1 ratio)
so the read locking is not really buying us much in the way of parallelism.

To avoid the read vs write contention, change the cache to use RCU locking on
the read side. To avoid needing to RCU free every single inode, use the built
in slab RCU freeing mechanism. This requires us to be able to detect lookups of
freed inodes, so enѕure that ever freed inode has an inode number of zero and
the XFS_IRECLAIM flag set. We already check the XFS_IRECLAIM flag in cache hit
lookup path, but also add a check for a zero inode number as well.

We canthen convert all the read locking lockups to use RCU read side locking
and hence remove all read side locking.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
Reviewed-by: Alex Elder <aelder@xxxxxxx>
---
 fs/xfs/linux-2.6/xfs_iops.c    |    7 +++++-
 fs/xfs/linux-2.6/xfs_sync.c    |   13 +++++++++--
 fs/xfs/quota/xfs_qm_syscalls.c |    3 ++
 fs/xfs/xfs_iget.c              |   44 ++++++++++++++++++++++++++++++---------
 fs/xfs/xfs_inode.c             |   22 ++++++++++++-------
 5 files changed, 67 insertions(+), 22 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_iops.c b/fs/xfs/linux-2.6/xfs_iops.c
index 8b46867..909bd9c 100644
--- a/fs/xfs/linux-2.6/xfs_iops.c
+++ b/fs/xfs/linux-2.6/xfs_iops.c
@@ -757,6 +757,8 @@ xfs_diflags_to_iflags(
  * We don't use the VFS inode hash for lookups anymore, so make the inode look
  * hashed to the VFS by faking it. This avoids needing to touch inode hash
  * locks in this path, but makes the VFS believe the inode is validly hashed.
+ * We initialise i_state and i_hash under the i_lock so that we follow the same
+ * setup rules that the rest of the VFS follows.
  */
 void
 xfs_setup_inode(
@@ -765,10 +767,13 @@ xfs_setup_inode(
        struct inode            *inode = &ip->i_vnode;
 
        inode->i_ino = ip->i_ino;
+
+       spin_lock(&inode->i_lock);
        inode->i_state = I_NEW;
+       hlist_nulls_add_fake(&inode->i_hash);
+       spin_unlock(&inode->i_lock);
 
        inode_sb_list_add(inode);
-       hlist_nulls_add_fake(&inode->i_hash);
 
        inode->i_mode   = ip->i_d.di_mode;
        inode->i_nlink  = ip->i_d.di_nlink;
diff --git a/fs/xfs/linux-2.6/xfs_sync.c b/fs/xfs/linux-2.6/xfs_sync.c
index afb0d7c..9a53cc9 100644
--- a/fs/xfs/linux-2.6/xfs_sync.c
+++ b/fs/xfs/linux-2.6/xfs_sync.c
@@ -53,6 +53,10 @@ xfs_inode_ag_walk_grab(
 {
        struct inode            *inode = VFS_I(ip);
 
+       /* check for stale RCU freed inode */
+       if (!ip->i_ino)
+               return ENOENT;
+
        /* nothing to sync during shutdown */
        if (XFS_FORCED_SHUTDOWN(ip->i_mount))
                return EFSCORRUPTED;
@@ -98,12 +102,12 @@ restart:
                int             error = 0;
                int             i;
 
-               read_lock(&pag->pag_ici_lock);
+               rcu_read_lock();
                nr_found = radix_tree_gang_lookup(&pag->pag_ici_root,
                                        (void **)batch, first_index,
                                        XFS_LOOKUP_BATCH);
                if (!nr_found) {
-                       read_unlock(&pag->pag_ici_lock);
+                       rcu_read_unlock();
                        break;
                }
 
@@ -129,7 +133,7 @@ restart:
                }
 
                /* unlock now we've grabbed the inodes. */
-               read_unlock(&pag->pag_ici_lock);
+               rcu_read_unlock();
 
                for (i = 0; i < nr_found; i++) {
                        if (!batch[i])
@@ -639,6 +643,9 @@ xfs_reclaim_inode_grab(
        struct xfs_inode        *ip,
        int                     flags)
 {
+       /* check for stale RCU freed inode */
+       if (!ip->i_ino)
+               return 1;
 
        /*
         * do some unlocked checks first to avoid unnecceary lock traffic.
diff --git a/fs/xfs/quota/xfs_qm_syscalls.c b/fs/xfs/quota/xfs_qm_syscalls.c
index bdebc18..8b207fc 100644
--- a/fs/xfs/quota/xfs_qm_syscalls.c
+++ b/fs/xfs/quota/xfs_qm_syscalls.c
@@ -875,6 +875,9 @@ xfs_dqrele_inode(
        struct xfs_perag        *pag,
        int                     flags)
 {
+       if (!ip->i_ino)
+               return ENOENT;
+
        /* skip quota inodes */
        if (ip == ip->i_mount->m_quotainfo->qi_uquotaip ||
            ip == ip->i_mount->m_quotainfo->qi_gquotaip) {
diff --git a/fs/xfs/xfs_iget.c b/fs/xfs/xfs_iget.c
index 18991a9..edeb918 100644
--- a/fs/xfs/xfs_iget.c
+++ b/fs/xfs/xfs_iget.c
@@ -69,6 +69,7 @@ xfs_inode_alloc(
        ASSERT(atomic_read(&ip->i_pincount) == 0);
        ASSERT(!spin_is_locked(&ip->i_flags_lock));
        ASSERT(completion_done(&ip->i_flush));
+       ASSERT(ip->i_ino == 0);
 
        mrlock_init(&ip->i_iolock, MRLOCK_BARRIER, "xfsio", ip->i_ino);
 
@@ -86,9 +87,6 @@ xfs_inode_alloc(
        ip->i_new_size = 0;
        ip->i_dirty_releases = 0;
 
-       /* prevent anyone from using this yet */
-       VFS_I(ip)->i_state = I_NEW;
-
        return ip;
 }
 
@@ -135,6 +133,16 @@ xfs_inode_free(
        ASSERT(!spin_is_locked(&ip->i_flags_lock));
        ASSERT(completion_done(&ip->i_flush));
 
+       /*
+        * because we use SLAB_DESTROY_BY_RCU freeing, ensure the inode
+        * always appears to be reclaimed with an invalid inode number
+        * when in the free state. The ip->i_flags_lock provides the barrier
+        * against lookup races.
+        */
+       spin_lock(&ip->i_flags_lock);
+       ip->i_flags = XFS_IRECLAIM;
+       ip->i_ino = 0;
+       spin_unlock(&ip->i_flags_lock);
        kmem_zone_free(xfs_inode_zone, ip);
 }
 
@@ -146,12 +154,28 @@ xfs_iget_cache_hit(
        struct xfs_perag        *pag,
        struct xfs_inode        *ip,
        int                     flags,
-       int                     lock_flags) __releases(pag->pag_ici_lock)
+       int                     lock_flags) __releases(RCU)
 {
        struct inode            *inode = VFS_I(ip);
        struct xfs_mount        *mp = ip->i_mount;
        int                     error;
 
+       /*
+        * check for re-use of an inode within an RCU grace period due to the
+        * radix tree nodes not being updated yet. We monitor for this by
+        * setting the inode number to zero before freeing the inode structure.
+        * We don't need to recheck this after taking the i_flags_lock because
+        * the check against XFS_IRECLAIM will catch a freed inode.
+        */
+       if (ip->i_ino == 0) {
+               trace_xfs_iget_skip(ip);
+               XFS_STATS_INC(xs_ig_frecycle);
+               rcu_read_unlock();
+               /* Expire the grace period so we don't trip over it again. */
+               synchronize_rcu();
+               return EAGAIN;
+       }
+
        spin_lock(&ip->i_flags_lock);
 
        /*
@@ -195,7 +219,7 @@ xfs_iget_cache_hit(
                ip->i_flags |= XFS_IRECLAIM;
 
                spin_unlock(&ip->i_flags_lock);
-               read_unlock(&pag->pag_ici_lock);
+               rcu_read_unlock();
 
                error = -inode_init_always(mp->m_super, inode);
                if (error) {
@@ -203,7 +227,7 @@ xfs_iget_cache_hit(
                         * Re-initializing the inode failed, and we are in deep
                         * trouble.  Try to re-add it to the reclaim list.
                         */
-                       read_lock(&pag->pag_ici_lock);
+                       rcu_read_lock();
                        spin_lock(&ip->i_flags_lock);
 
                        ip->i_flags &= ~XFS_INEW;
@@ -231,7 +255,7 @@ xfs_iget_cache_hit(
 
                /* We've got a live one. */
                spin_unlock(&ip->i_flags_lock);
-               read_unlock(&pag->pag_ici_lock);
+               rcu_read_unlock();
                trace_xfs_iget_hit(ip);
        }
 
@@ -245,7 +269,7 @@ xfs_iget_cache_hit(
 
 out_error:
        spin_unlock(&ip->i_flags_lock);
-       read_unlock(&pag->pag_ici_lock);
+       rcu_read_unlock();
        return error;
 }
 
@@ -376,7 +400,7 @@ xfs_iget(
 
 again:
        error = 0;
-       read_lock(&pag->pag_ici_lock);
+       rcu_read_lock();
        ip = radix_tree_lookup(&pag->pag_ici_root, agino);
 
        if (ip) {
@@ -384,7 +408,7 @@ again:
                if (error)
                        goto out_error_or_again;
        } else {
-               read_unlock(&pag->pag_ici_lock);
+               rcu_read_unlock();
                XFS_STATS_INC(xs_ig_missed);
 
                error = xfs_iget_cache_miss(mp, pag, tp, ino, &ip,
diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index 108c7a0..25becb1 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -2000,13 +2000,14 @@ xfs_ifree_cluster(
                 */
                for (i = 0; i < ninodes; i++) {
 retry:
-                       read_lock(&pag->pag_ici_lock);
+                       rcu_read_lock();
                        ip = radix_tree_lookup(&pag->pag_ici_root,
                                        XFS_INO_TO_AGINO(mp, (inum + i)));
 
                        /* Inode not in memory or stale, nothing to do */
-                       if (!ip || xfs_iflags_test(ip, XFS_ISTALE)) {
-                               read_unlock(&pag->pag_ici_lock);
+                       if (!ip || !ip->i_ino ||
+                           xfs_iflags_test(ip, XFS_ISTALE)) {
+                               rcu_read_unlock();
                                continue;
                        }
 
@@ -2019,11 +2020,11 @@ retry:
                         */
                        if (ip != free_ip &&
                            !xfs_ilock_nowait(ip, XFS_ILOCK_EXCL)) {
-                               read_unlock(&pag->pag_ici_lock);
+                               rcu_read_unlock();
                                delay(1);
                                goto retry;
                        }
-                       read_unlock(&pag->pag_ici_lock);
+                       rcu_read_unlock();
 
                        xfs_iflock(ip);
                        xfs_iflags_set(ip, XFS_ISTALE);
@@ -2629,7 +2630,7 @@ xfs_iflush_cluster(
 
        mask = ~(((XFS_INODE_CLUSTER_SIZE(mp) >> mp->m_sb.sb_inodelog)) - 1);
        first_index = XFS_INO_TO_AGINO(mp, ip->i_ino) & mask;
-       read_lock(&pag->pag_ici_lock);
+       rcu_read_lock();
        /* really need a gang lookup range call here */
        nr_found = radix_tree_gang_lookup(&pag->pag_ici_root, (void**)ilist,
                                        first_index, inodes_per_cluster);
@@ -2640,6 +2641,11 @@ xfs_iflush_cluster(
                iq = ilist[i];
                if (iq == ip)
                        continue;
+
+               /* check we've got a valid inode */
+               if (!iq->i_ino)
+                       continue;
+
                /* if the inode lies outside this cluster, we're done. */
                if ((XFS_INO_TO_AGINO(mp, iq->i_ino) & mask) != first_index)
                        break;
@@ -2692,7 +2698,7 @@ xfs_iflush_cluster(
        }
 
 out_free:
-       read_unlock(&pag->pag_ici_lock);
+       rcu_read_unlock();
        kmem_free(ilist);
 out_put:
        xfs_perag_put(pag);
@@ -2704,7 +2710,7 @@ cluster_corrupt_out:
         * Corruption detected in the clustering loop.  Invalidate the
         * inode buffer and shut down the filesystem.
         */
-       read_unlock(&pag->pag_ici_lock);
+       rcu_read_unlock();
        /*
         * Clean up the buffer.  If it was B_DELWRI, just release it --
         * brelse can handle it with no problems.  If not, shut down the
-- 
1.7.2.3

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