Btrfs: change reserved_extents to an atomic_t

We track delayed allocation per inodes via 2 counters, one is
outstanding_extents and reserved_extents.  Outstanding_extents is already an
atomic_t, but reserved_extents is not and is protected by a spinlock.  So
convert this to an atomic_t and instead of using a spinlock, use atomic_cmpxchg
when releasing delalloc bytes.  This makes our inode 72 bytes smaller, and
reduces locking overhead (albiet it was minimal to begin with).  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

1 files changed, 26 insertions, 16 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 7b3089b5c2df..27376c97d85f 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3996,6 +3996,7 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
         struct btrfs_block_rsv *block_rsv = &root->fs_info->delalloc_block_rsv;
         u64 to_reserve;
         int nr_extents;
+        int reserved_extents;
         int ret;
 
         if (btrfs_transaction_in_commit(root->fs_info))
@@ -4003,25 +4004,24 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 
         num_bytes = ALIGN(num_bytes, root->sectorsize);
 
-        spin_lock(&BTRFS_I(inode)->accounting_lock);
         nr_extents = atomic_read(&BTRFS_I(inode)->outstanding_extents) + 1;
-        if (nr_extents > BTRFS_I(inode)->reserved_extents) {
-                nr_extents -= BTRFS_I(inode)->reserved_extents;
+        reserved_extents = atomic_read(&BTRFS_I(inode)->reserved_extents);
+
+        if (nr_extents > reserved_extents) {
+                nr_extents -= reserved_extents;
                 to_reserve = calc_trans_metadata_size(root, nr_extents);
         } else {
                 nr_extents = 0;
                 to_reserve = 0;
         }
-        spin_unlock(&BTRFS_I(inode)->accounting_lock);
+
         to_reserve += calc_csum_metadata_size(inode, num_bytes);
         ret = reserve_metadata_bytes(NULL, root, block_rsv, to_reserve, 1);
         if (ret)
                 return ret;
 
-        spin_lock(&BTRFS_I(inode)->accounting_lock);
-        BTRFS_I(inode)->reserved_extents += nr_extents;
+        atomic_add(nr_extents, &BTRFS_I(inode)->reserved_extents);
         atomic_inc(&BTRFS_I(inode)->outstanding_extents);
-        spin_unlock(&BTRFS_I(inode)->accounting_lock);
 
         block_rsv_add_bytes(block_rsv, to_reserve, 1);
 
@@ -4036,20 +4036,30 @@ void btrfs_delalloc_release_metadata(struct inode *inode, u64 num_bytes)
         struct btrfs_root *root = BTRFS_I(inode)->root;
         u64 to_free;
         int nr_extents;
+        int reserved_extents;
 
         num_bytes = ALIGN(num_bytes, root->sectorsize);
         atomic_dec(&BTRFS_I(inode)->outstanding_extents);
         WARN_ON(atomic_read(&BTRFS_I(inode)->outstanding_extents) < 0);
 
-        spin_lock(&BTRFS_I(inode)->accounting_lock);
-        nr_extents = atomic_read(&BTRFS_I(inode)->outstanding_extents);
-        if (nr_extents < BTRFS_I(inode)->reserved_extents) {
-                nr_extents = BTRFS_I(inode)->reserved_extents - nr_extents;
-                BTRFS_I(inode)->reserved_extents -= nr_extents;
-        } else {
-                nr_extents = 0;
-        }
-        spin_unlock(&BTRFS_I(inode)->accounting_lock);
+        reserved_extents = atomic_read(&BTRFS_I(inode)->reserved_extents);
+        do {
+                int old, new;
+
+                nr_extents = atomic_read(&BTRFS_I(inode)->outstanding_extents);
+                if (nr_extents >= reserved_extents) {
+                        nr_extents = 0;
+                        break;
+                }
+                old = reserved_extents;
+                nr_extents = reserved_extents - nr_extents;
+                new = reserved_extents - nr_extents;
+                old = atomic_cmpxchg(&BTRFS_I(inode)->reserved_extents,
+                                     reserved_extents, new);
+                if (likely(old == reserved_extents))
+                        break;
+                reserved_extents = old;
+        } while (1);
 
         to_free = calc_csum_metadata_size(inode, num_bytes);
         if (nr_extents > 0)