linux-kernel - [PATCH 3.14 61/77] fs: buffer: do not use unnecessary atomic operations when discarding buffers

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Message-Id: <20150128012747.796039292@linuxfoundation.org>
Date:	Tue, 27 Jan 2015 17:27:39 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Theodore Tso <tytso@....edu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 3.14 61/77] fs: buffer: do not use unnecessary atomic operations when discarding buffers

3.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mel Gorman <mgorman@...e.de>

commit e7470ee89f003634a88e7b5e5a7b65b3025987de upstream.

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.

[akpm@...ux-foundation.org: move BUFFER_FLAGS_DISCARD into the .c file]
Signed-off-by: Mel Gorman <mgorman@...e.de>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Jan Kara <jack@...e.cz>
Cc: Michal Hocko <mhocko@...e.cz>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Theodore Ts'o <tytso@....edu>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Rik van Riel <riel@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@...e.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/buffer.c |   21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1485,16 +1485,27 @@ EXPORT_SYMBOL(set_bh_page);
 /*
  * Called when truncating a buffer on a page completely.
  */
+
+/* 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)
+
 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);
 }

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