[PATCH 17/19] fs: buffer: Do not use unnecessary atomic operations when discarding buffers

[Mel Gorman <mgorman@suse.de>]

Discarding buffers uses a bunch of atomic operations when discarding buffers
because ...... I can't think of a reason. Use a cmpxchg loop to clear all the
necessary flags. In most (all?) cases this will be a single atomic operations.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 fs/buffer.c                 | 14 +++++++++-----
 include/linux/buffer_head.h |  5 +++++
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 9ddb9fc..e80012d 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1485,14 +1485,18 @@ EXPORT_SYMBOL(set_bh_page);
  */
 static void discard_buffer(struct buffer_head * bh)
 {
+	unsigned long b_state, b_state_old;
+
 	lock_buffer(bh);
 	clear_buffer_dirty(bh);
 	bh->b_bdev = NULL;
-	clear_buffer_mapped(bh);
-	clear_buffer_req(bh);
-	clear_buffer_new(bh);
-	clear_buffer_delay(bh);
-	clear_buffer_unwritten(bh);
+	b_state = bh->b_state;
+	for (;;) {
+		b_state_old = cmpxchg(&bh->b_state, b_state, (b_state & ~BUFFER_FLAGS_DISCARD));
+		if (b_state_old == b_state)
+			break;
+		b_state = b_state_old;
+	}
 	unlock_buffer(bh);
 }
 
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index c40302f..95f565a 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -77,6 +77,11 @@ struct buffer_head {
 	atomic_t b_count;		/* users using this buffer_head */
 };
 
+/* Bits that are cleared during an invalidate */
+#define BUFFER_FLAGS_DISCARD \
+	(1 << BH_Mapped | 1 << BH_New | 1 << BH_Req | \
+	 1 << BH_Delay | 1 << BH_Unwritten)
+
 /*
  * macro tricks to expand the set_buffer_foo(), clear_buffer_foo()
  * and buffer_foo() functions.
-- 
1.8.4.5

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