[PATCH 10/24] btrfs: count extents before taking inode's spinlock when reserving metadata

[fdmanana@kernel.org]

From: Filipe Manana <fdmanana@suse.com>

When reserving metadata space for delalloc (and direct IO too), at
btrfs_delalloc_reserve_metadata(), there's no need to count the number of
extents while holding the inode's spinlock, since that does not require
access to any field of the inode.

This section of code can be called concurrently, when we have direct IO
writes against different file ranges that don't increase the inode's
i_size, so it's beneficial to shorten the critical section by counting
the number of extents before taking the inode's spinlock.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/delalloc-space.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/delalloc-space.c b/fs/btrfs/delalloc-space.c
index 7ddb1d104e8e..427abaf608b8 100644
--- a/fs/btrfs/delalloc-space.c
+++ b/fs/btrfs/delalloc-space.c
@@ -358,8 +358,8 @@ int btrfs_delalloc_reserve_metadata(struct btrfs_inode *inode, u64 num_bytes,
 	 * racing with an ordered completion or some such that would think it
 	 * needs to free the reservation we just made.
 	 */
-	spin_lock(&inode->lock);
 	nr_extents = count_max_extents(fs_info, num_bytes);
+	spin_lock(&inode->lock);
 	btrfs_mod_outstanding_extents(inode, nr_extents);
 	inode->csum_bytes += disk_num_bytes;
 	btrfs_calculate_inode_block_rsv_size(fs_info, inode);
-- 
2.34.1