linux-kernel - [PATCH] btrfs: optimize barrier usage for Rmw atomics

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Date:   Wed, 29 Jan 2020 10:03:24 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     dsterba@...e.com
Cc:     nborisov@...e.com, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, dave@...olabs.net,
        Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH] btrfs: optimize barrier usage for Rmw atomics

Use smp_mb__after_atomic() instead of smp_mb() and avoid the
unnecessary barrier for non LL/SC architectures, such as x86.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 fs/btrfs/btrfs_inode.h | 2 +-
 fs/btrfs/file.c        | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index 4e12a477d32e..54e0d2ae22cc 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -325,7 +325,7 @@ struct btrfs_dio_private {
 static inline void btrfs_inode_block_unlocked_dio(struct btrfs_inode *inode)
 {
 	set_bit(BTRFS_INODE_READDIO_NEED_LOCK, &inode->runtime_flags);
-	smp_mb();
+	smp_mb__after_atomic();
 }
 
 static inline void btrfs_inode_resume_unlocked_dio(struct btrfs_inode *inode)
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index a16da274c9aa..ea79ab068079 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2143,7 +2143,7 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 	}
 	atomic_inc(&root->log_batch);
 
-	smp_mb();
+	smp_mb__after_atomic();
 	if (btrfs_inode_in_log(BTRFS_I(inode), fs_info->generation) ||
 	    BTRFS_I(inode)->last_trans <= fs_info->last_trans_committed) {
 		/*
-- 
2.16.4