linux-kernel - Re: [PATCH] Add a read memory barrier to wait_on

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Date:   Sun, 31 Jul 2022 14:00:16 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        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>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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] Add a read memory barrier to wait_on_buffer

Hi Mikulas,

On Sun, 31 Jul 2022 at 13:43, 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 a memory barrier.
> Consequently, if someone calls sb_bread and then reads the buffer data,
> the read of buffer data may be speculatively executed before
> wait_on_buffer(bh) and it may return invalid data.
>

This has little to do with speculation, so better to drop this S bomb
from your commit message. This is about concurrency and weak memory
ordering.

> Also, there is this pattern present several times:
>         wait_on_buffer(bh);
>         if (!buffer_uptodate(bh))
>                 err = -EIO;
> It may be possible that buffer_uptodate is executed before wait_on_buffer
> and it may return spurious error.
>
> Fix these bugs by adding a read memory barrier to wait_on_buffer().
>
> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> Cc: stable@...r.kernel.org
>
> Index: linux-2.6/include/linux/buffer_head.h
> ===================================================================
> --- linux-2.6.orig/include/linux/buffer_head.h
> +++ linux-2.6/include/linux/buffer_head.h
> @@ -353,6 +353,11 @@ static inline void wait_on_buffer(struct
>         might_sleep();
>         if (buffer_locked(bh))
>                 __wait_on_buffer(bh);
> +       /*
> +        * Make sure that the following accesses to buffer state or buffer data
> +        * are not reordered with buffer_locked(bh).
> +        */
> +       smp_rmb();
>  }
>
>  static inline int trylock_buffer(struct buffer_head *bh)
>

This doesn't seem like a very robust fix to me, tbh - I suppose this
makes the symptom you encountered go away, but the underlying issue
remains afaict.

Given that the lock and uptodate fields etc are just bits in a
bitfield, wouldn't it be better to use cmpxchg() with acquire/release
semantics (as appropriate) to manage these bits?