linux-kernel - Re: [PATCH v6] add barriers to buffer_uptodate and set_buffer

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Date:   Tue, 9 Aug 2022 15:06:25 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.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 v6] add barriers to buffer_uptodate and set_buffer_uptodate

On Tue, Aug 9, 2022 at 11:32 AM Mikulas Patocka <mpatocka@...hat.com> 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 adding a memory barrier to set_buffer_uptodate and an
> acquire barrier to buffer_uptodate (in a similar way as
> folio_test_uptodate and folio_mark_uptodate).

Ok, I've applied this to my tree.

I still feel that we should probably take a long look at having the
proper "acquire/release" uses everywhere for the buffer / page / folio
flags, but that wouldn't really work for backporting to stable, so I
think that's a "future fixes/cleanup" thing.

Thanks,
              Linus