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SubjectRe: [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc
On Tue, Jun 25, 2019 at 11:24 AM Nathan Chancellor
<natechancellor@gmail.com> 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@google.com>

Björn 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@gmail.com>
> ---
> 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

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