Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
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

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.