From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:51064 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754381AbeBNIxm (ORCPT ); Wed, 14 Feb 2018 03:53:42 -0500 Received: from relay1.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id E343AAC39 for ; Wed, 14 Feb 2018 08:53:40 +0000 (UTC) From: Nikolay Borisov To: linux-btrfs@vger.kernel.org Cc: Nikolay Borisov Subject: [PATCH] btrfs: Remove redundant memory barriers Date: Wed, 14 Feb 2018 10:53:36 +0200 Message-Id: <1518598416-13069-1-git-send-email-nborisov@suse.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: Using any kind of memory barriers around atomic operations which have a return value is redundant, since those operations themselves are fully ordered. atomic_t.txt states: - RMW operations that have a return value are fully ordered; Fully ordered primitives are ordered against everything prior and everything subsequent. Therefore a fully ordered primitive is like having an smp_mb() before and an smp_mb() after the primitive. Given this let's replace the extra memory barriers with comments. Signed-off-by: Nikolay Borisov --- fs/btrfs/inode.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 8997e39c8e37..b6e4288010fa 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -8321,13 +8321,13 @@ static void btrfs_end_dio_bio(struct bio *bio) err = dip->subio_endio(dip->inode, btrfs_io_bio(bio), err); if (err) { - dip->errors = 1; - /* - * before atomic variable goto zero, we must make sure - * dip->errors is perceived to be set. + * We want to perceive the errors flag being set before + * decrementing the reference count. We don't need a barrier + * since atomic operations with a return value are fully + * ordered as per atomic_t.txt */ - smp_mb__before_atomic(); + dip->errors = 1; } /* if there are more bios still pending for this dio, just exit */ @@ -8515,10 +8515,11 @@ static int btrfs_submit_direct_hook(struct btrfs_dio_private *dip) out_err: dip->errors = 1; /* - * before atomic variable goto zero, we must - * make sure dip->errors is perceived to be set. + * Before atomic variable goto zero, we must make sure dip->errors is + * perceived to be set. This ordering is ensured by the fact that an + * atomic operations with a return value are fully ordered as per + * atomic_t.txt */ - smp_mb__before_atomic(); if (atomic_dec_and_test(&dip->pending_bios)) bio_io_error(dip->orig_bio); -- 2.7.4