Re: [PATCH] Add a read memory barrier to wait_on_buffer

[Ard Biesheuvel]

Hi Mikulas,

On Sun, 31 Jul 2022 at 13:43, Mikulas Patocka <mpatocka@xxxxxxxxxx> 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@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
>
> 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?