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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 9 Aug 2022 15:06:25 -0700
Message-ID: <CAHk-=wiivNAj+jdXUdo91PQgh__vtE3WCFgufV0yVu7mWAKQWg@mail.gmail.com>
Subject: Re: [PATCH v6] add barriers to buffer_uptodate and set_buffer_uptodate
To:     Mikulas Patocka <mpatocka@redhat.com>
Cc:     Matthew Wilcox <willy@infradead.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
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On Tue, Aug 9, 2022 at 11:32 AM Mikulas Patocka <mpatocka@redhat.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