From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
Received: from vger.kernel.org (vger.kernel.org [23.128.96.18])
	by smtp.lore.kernel.org (Postfix) with ESMTP id 7882CC00140
	for <linux-kernel@archiver.kernel.org>; Mon,  8 Aug 2022 14:41:08 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S243257AbiHHOlC (ORCPT <rfc822;linux-kernel@archiver.kernel.org>);
        Mon, 8 Aug 2022 10:41:02 -0400
Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35932 "EHLO
        lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S234788AbiHHOkz (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 8 Aug 2022 10:40:55 -0400
Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1])
        by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D0A4E2A1;
        Mon,  8 Aug 2022 07:40:54 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed;
        d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version:
        References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To:
        Content-Transfer-Encoding:Content-ID:Content-Description;
        bh=LTOFeZ2UuJ/bdyef4DVRV/zTuITTtVz6bqJzh2YiCq8=; b=Xhuv38YLuPdi7xvcT5h+Oexcre
        Dm3J16/unTMb0cJnSTEPwVCiLuGs2CpnhiLLtBUJ8eM6t4HbjYZdhILemW/VmokQwR2jVw+4HQsAL
        oytx8f2GIqfBUEnXwUFLxyraiXK823+OKDgwh+TdIlfKF78UvV8hwKUG2i3v+1G2icu965TA7nims
        DQBWYtPkCjc6q8I6rz1ZHJuD4/A/CZAf6/6UtBXXE8r0aIHr6+vxAGo1tYuom9qfJWS6YveE3y6nc
        8RT0kLxafsNCz8zKb/wXcfHA+OIPlMWKRsuFfFJ3WHLe8WrAA3TL5C8lgmf0hEeztHCX8zG7UIiTe
        V4bAWQNg==;
Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux))
        id 1oL3vq-00DzKo-EN; Mon, 08 Aug 2022 14:40:38 +0000
Date:   Mon, 8 Aug 2022 15:40:38 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Mikulas Patocka <mpatocka@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v5] add barriers to buffer functions
Message-ID: <YvEgZuSdv8XHtkJg@casper.infradead.org>
References: <alpine.LRH.2.02.2207311639360.21350@file01.intranet.prod.int.rdu2.redhat.com>
 <CAHk-=wjA8HBrVqAqAetUvwNr=hcvhfnO7oMrOAd4V8bbSqokNA@mail.gmail.com>
 <alpine.LRH.2.02.2208010628510.22006@file01.intranet.prod.int.rdu2.redhat.com>
 <alpine.LRH.2.02.2208010642220.22006@file01.intranet.prod.int.rdu2.redhat.com>
 <YuflGG60pHiXp2z/@casper.infradead.org>
 <alpine.LRH.2.02.2208011040190.27101@file01.intranet.prod.int.rdu2.redhat.com>
 <YuyNE5c06WStxQ2z@casper.infradead.org>
 <alpine.LRH.2.02.2208070732160.30857@file01.intranet.prod.int.rdu2.redhat.com>
 <Yu/RJtoJPhkWXIdP@casper.infradead.org>
 <alpine.LRH.2.02.2208080928580.8160@file01.intranet.prod.int.rdu2.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.LRH.2.02.2208080928580.8160@file01.intranet.prod.int.rdu2.redhat.com>
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Aug 08, 2022 at 10:26:10AM -0400, Mikulas Patocka wrote:
> On Sun, 7 Aug 2022, Matthew Wilcox wrote:
> > > +static __always_inline void set_buffer_locked(struct buffer_head *bh)
> > > +{
> > > +	set_bit(BH_Lock, &bh->b_state);
> > > +}
> > > +
> > > +static __always_inline int buffer_locked(const struct buffer_head *bh)
> > > +{
> > > +	bool ret = test_bit(BH_Lock, &bh->b_state);
> > > +	/*
> > > +	 * pairs with smp_mb__after_atomic in unlock_buffer
> > > +	 */
> > > +	if (!ret)
> > > +		smp_acquire__after_ctrl_dep();
> > > +	return ret;
> > > +}
> > 
> > Are there places that think that lock/unlock buffer implies a memory
> > barrier?
> 
> There's this in fs/reiserfs:
> 
> if (!buffer_dirty(bh) && !buffer_locked(bh)) {
> 	reiserfs_free_jh(bh); <--- this could be moved before buffer_locked

It might be better to think of buffer_locked() as
buffer_someone_has_exclusive_access().  I can't see the problem with
moving the reads in reiserfs_free_jh() before the read of buffer_locked.

> if (buffer_locked((journal->j_header_bh))) {
> 	...
> }
> journal->j_last_flush_trans_id = trans_id;
> journal->j_first_unflushed_offset = offset;
> jh = (struct reiserfs_journal_header *)(journal->j_header_bh->b_data); <--- this could be moved before buffer_locked

I don't think b_data is going to be changed while someone else holds
the buffer locked.  That's initialised by set_bh_page(), which is an
initialisation-time thing, before the BH is visible to any other thread.