Re: [PATCH v6] add barriers to buffer_uptodate and set_buffer_uptodate

[Matthew Wilcox]

On Tue, Aug 09, 2022 at 02:32:13PM -0400, Mikulas Patocka wrote:
> From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> 
> 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).
> 
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

> 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
> @@ -117,7 +117,6 @@ static __always_inline int test_clear_bu
>   * of the form "mark_buffer_foo()".  These are higher-level functions which
>   * do something in addition to setting a b_state bit.
>   */
> -BUFFER_FNS(Uptodate, uptodate)
>  BUFFER_FNS(Dirty, dirty)
>  TAS_BUFFER_FNS(Dirty, dirty)
>  BUFFER_FNS(Lock, locked)
> @@ -135,6 +134,30 @@ BUFFER_FNS(Meta, meta)
>  BUFFER_FNS(Prio, prio)
>  BUFFER_FNS(Defer_Completion, defer_completion)
>  
> +static __always_inline void set_buffer_uptodate(struct buffer_head *bh)
> +{
> +	/*
> +	 * make it consistent with folio_mark_uptodate
> +	 * pairs with smp_load_acquire in buffer_uptodate
> +	 */
> +	smp_mb__before_atomic();
> +	set_bit(BH_Uptodate, &bh->b_state);
> +}
> +
> +static __always_inline void clear_buffer_uptodate(struct buffer_head *bh)
> +{
> +	clear_bit(BH_Uptodate, &bh->b_state);
> +}
> +
> +static __always_inline int buffer_uptodate(const struct buffer_head *bh)
> +{
> +	/*
> +	 * make it consistent with folio_test_uptodate
> +	 * pairs with smp_mb__before_atomic in set_buffer_uptodate
> +	 */
> +	return (smp_load_acquire(&bh->b_state) & (1UL << BH_Uptodate)) != 0;
> +}
> +
>  #define bh_offset(bh)		((unsigned long)(bh)->b_data & ~PAGE_MASK)
>  
>  /* If we *know* page->private refers to buffer_heads */
>