Re: [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc

[Nick Desaulniers]

On Tue, Jun 25, 2019 at 11:24 AM Nathan Chancellor
<natechancellor@xxxxxxxxx> wrote:
>
> Clang warns:
>
> In file included from net/xdp/xsk_queue.c:10:
> net/xdp/xsk_queue.h:292:2: warning: expression result unused
> [-Wunused-value]
>         WRITE_ONCE(q->ring->producer, q->prod_tail);
>         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> include/linux/compiler.h:284:6: note: expanded from macro 'WRITE_ONCE'
>         __u.__val;                                      \
>         ~~~ ^~~~~
> 1 warning generated.
>
> The q->prod_tail assignment has a comma at the end, not a semi-colon.
> Fix that so clang no longer warns and everything works as expected.

oh no! -Wunderhanded-C-contest-style-use-of-comma-operator strikes again!
Great find and thanks for the fix.
Acked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

BjÃrn and Alexei should carefully review.  Because WRITE_ONCE is a
macro that expands to a GNU C statement expression, which returns the
last statement, this code was previously assigning q->prod_tail to
itself, now it's assigning it to q->prod_head.  I assume that was not
intentional, but am unfamiliar with the code.

>
> Fixes: c497176cb2e4 ("xsk: add Rx receive functions and poll support")
> Link: https://github.com/ClangBuiltLinux/linux/issues/544
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> ---
>  net/xdp/xsk_queue.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
> index 88b9ae24658d..cba4a640d5e8 100644
> --- a/net/xdp/xsk_queue.h
> +++ b/net/xdp/xsk_queue.h
> @@ -288,7 +288,7 @@ static inline void xskq_produce_flush_desc(struct xsk_queue *q)
>         /* Order producer and data */
>         smp_wmb(); /* B, matches C */
>
> -       q->prod_tail = q->prod_head,
> +       q->prod_tail = q->prod_head;
>         WRITE_ONCE(q->ring->producer, q->prod_tail);
>  }
>
> --
> 2.22.0
>


-- 
Thanks,
~Nick Desaulniers