linux-kernel - Re: [PATCH v4 2/2] change buffer_locked, so that it has acquire semantics

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Message-ID: <YuflGG60pHiXp2z/@casper.infradead.org>
Date:   Mon, 1 Aug 2022 15:37:12 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] change buffer_locked, so that it has acquire
 semantics

On Mon, Aug 01, 2022 at 06:43:55AM -0400, Mikulas Patocka wrote:
> Let's have a look at this piece of code in __bread_slow:
> 	get_bh(bh);
> 	bh->b_end_io = end_buffer_read_sync;
> 	submit_bh(REQ_OP_READ, 0, bh);
> 	wait_on_buffer(bh);
> 	if (buffer_uptodate(bh))
> 		return bh;
> Neither wait_on_buffer nor buffer_uptodate contain any memory barrier.
> Consequently, if someone calls sb_bread and then reads the buffer data,
> the read of buffer data may be executed before wait_on_buffer(bh) on
> architectures with weak memory ordering and it may return invalid data.
> 
> Fix this bug by changing the function buffer_locked to have the acquire
> semantics - so that code that follows buffer_locked cannot be moved before
> it.

I think this is the wrong approach.  Instead, buffer_set_uptodate()
should have the smp_wmb() and buffer_uptodate should have the smp_rmb().
Just like the page flags.  As I said last night.