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* [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc
@ 2019-06-25 18:23 Nathan Chancellor
  2019-06-25 18:32 ` Nick Desaulniers
                   ` (3 more replies)
  0 siblings, 4 replies; 6+ messages in thread
From: Nathan Chancellor @ 2019-06-25 18:23 UTC (permalink / raw)
  To: Björn Töpel, Magnus Karlsson, David S. Miller,
	Alexei Starovoitov, Daniel Borkmann, Jakub Kicinski,
	Jesper Dangaard Brouer, John Fastabend
  Cc: netdev, bpf, xdp-newbies, linux-kernel, clang-built-linux,
	Nick Desaulniers, Nathan Huckleberry, Nathan Chancellor

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.

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


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc
  2019-06-25 18:23 [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc Nathan Chancellor
@ 2019-06-25 18:32 ` Nick Desaulniers
  2019-06-25 19:46 ` Jonathan Lemon
                   ` (2 subsequent siblings)
  3 siblings, 0 replies; 6+ messages in thread
From: Nick Desaulniers @ 2019-06-25 18:32 UTC (permalink / raw)
  To: Nathan Chancellor
  Cc: Björn Töpel, Magnus Karlsson, David S. Miller,
	Alexei Starovoitov, Daniel Borkmann, Jakub Kicinski,
	Jesper Dangaard Brouer, John Fastabend, netdev, bpf, xdp-newbies,
	LKML, clang-built-linux, Nathan Huckleberry

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

^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc
  2019-06-25 18:23 [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc Nathan Chancellor
  2019-06-25 18:32 ` Nick Desaulniers
@ 2019-06-25 19:46 ` Jonathan Lemon
  2019-06-25 20:49   ` Björn Töpel
  2019-06-26  1:13 ` Song Liu
  2019-06-26 14:30 ` Daniel Borkmann
  3 siblings, 1 reply; 6+ messages in thread
From: Jonathan Lemon @ 2019-06-25 19:46 UTC (permalink / raw)
  To: Nathan Chancellor
  Cc: Björn Töpel, Magnus Karlsson, David S. Miller,
	Alexei Starovoitov, Daniel Borkmann, Jakub Kicinski,
	Jesper Dangaard Brouer, John Fastabend, netdev, bpf, xdp-newbies,
	linux-kernel, clang-built-linux, Nick Desaulniers,
	Nathan Huckleberry



On 25 Jun 2019, at 11:23, Nathan Chancellor 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.
>
> 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>

Nice find.

Acked-by: Jonathan Lemon <jonathan.lemon@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

^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc
  2019-06-25 19:46 ` Jonathan Lemon
@ 2019-06-25 20:49   ` Björn Töpel
  0 siblings, 0 replies; 6+ messages in thread
From: Björn Töpel @ 2019-06-25 20:49 UTC (permalink / raw)
  To: Jonathan Lemon
  Cc: Nathan Chancellor, Björn Töpel, Magnus Karlsson,
	David S. Miller, Alexei Starovoitov, Daniel Borkmann,
	Jakub Kicinski, Jesper Dangaard Brouer, John Fastabend, Netdev,
	bpf, Xdp, LKML, clang-built-linux, Nick Desaulniers,
	Nathan Huckleberry

On Tue, 25 Jun 2019 at 22:04, Jonathan Lemon <jonathan.lemon@gmail.com> wrote:
>
>
>
> On 25 Jun 2019, at 11:23, Nathan Chancellor 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.
> >
> > 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>
>
> Nice find.
>
> Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
>

Yikes. Yes, nice find, indeed.

Acked-by: Björn Töpel <bjorn.topel@intel.com>

The broader question is "Why does it work at all?", which is an "oh no" moment.

The problematic functions are xsk_flush() and xsk_generic_rcv, where
xskq_produce_flush_desc() is inlined. On the test machine, the GCC
version is:

$ gcc --version
gcc (Ubuntu 7.4.0-1ubuntu1~18.04) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

I when I diff the output, both .lst and .o:

$ diff -u old.lst new.lst
--- old.lst     2019-06-25 22:10:57.709591605 +0200
+++ new.lst     2019-06-25 22:10:35.301359865 +0200
@@ -2480,7 +2480,7 @@
     1566:      48 8b 87 e0 02 00 00    mov    0x2e0(%rdi),%rax
 {
     156d:      48 89 e5                mov    %rsp,%rbp
-       q->prod_tail = q->prod_head,
+       q->prod_tail = q->prod_head;
     1570:      8b 50 18                mov    0x18(%rax),%edx
     1573:      89 50 1c                mov    %edx,0x1c(%rax)
        WRITE_ONCE(q->ring->producer, q->prod_tail);
@@ -2649,7 +2649,7 @@
     16fb:      83 40 24 01             addl   $0x1,0x24(%rax)
        xskq_produce_flush_desc(xs->rx);
     16ff:      49 8b 86 e0 02 00 00    mov    0x2e0(%r14),%rax
-       q->prod_tail = q->prod_head,
+       q->prod_tail = q->prod_head;
     1706:      8b 50 18                mov    0x18(%rax),%edx
        xs->sk.sk_data_ready(&xs->sk);
     1709:      4c 89 f7                mov    %r14,%rdi

$ diff -u <(gdb -batch -ex 'file old.o' -ex 'disassemble xsk_flush')
<(gdb -batch -ex 'file new.o' -ex 'disassemble xsk_flush') && echo
"Whew"
Whew

$ diff -u <(gdb -batch -ex 'file old.o' -ex 'disassemble
xsk_generic_rcv') <(gdb -batch -ex 'file new.o' -ex 'disassemble
xsk_generic_rcv') && echo "Whew"
Whew

struct xsk_queue {
        u64                        chunk_mask;           /*     0   0x8 */
        u64                        size;                 /*   0x8   0x8 */
        u32                        ring_mask;            /*  0x10   0x4 */
        u32                        nentries;             /*  0x14   0x4 */
        u32                        prod_head;            /*  0x18   0x4 */
        u32                        prod_tail;            /*  0x1c   0x4 */
        u32                        cons_head;            /*  0x20   0x4 */
        u32                        cons_tail;            /*  0x24   0x4 */
        struct xdp_ring *          ring;                 /*  0x28   0x8 */
        u64                        invalid_descs;        /*  0x30   0x8 */

        /* size: 56, cachelines: 1, members: 10 */
        /* last cacheline: 56 bytes */
};

So, it appears that the generated code is equal, both in xsk_flush()
and xsk_generic_rcv() where flush was inlined. I'll be digging into
more GCC versions, and observe the generated code.

Regardless, this was a really good find. Thank you very much! Clang is
added to my kernel build workflow from now on...


Björn



>
> > ---
> >  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

^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc
  2019-06-25 18:23 [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc Nathan Chancellor
  2019-06-25 18:32 ` Nick Desaulniers
  2019-06-25 19:46 ` Jonathan Lemon
@ 2019-06-26  1:13 ` Song Liu
  2019-06-26 14:30 ` Daniel Borkmann
  3 siblings, 0 replies; 6+ messages in thread
From: Song Liu @ 2019-06-26  1:13 UTC (permalink / raw)
  To: Nathan Chancellor
  Cc: Björn Töpel, Magnus Karlsson, David S. Miller,
	Alexei Starovoitov, Daniel Borkmann, Jakub Kicinski,
	Jesper Dangaard Brouer, John Fastabend, Networking, bpf,
	xdp-newbies, open list, clang-built-linux, Nick Desaulniers,
	Nathan Huckleberry

On Tue, Jun 25, 2019 at 12:54 PM 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.
>
> 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>

Cc: <stable@vger.kernel.org> # v4.18+
Acked-by: Song Liu <songliubraving@fb.com>

Thanks for the fix!


> ---
>  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
>

^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc
  2019-06-25 18:23 [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc Nathan Chancellor
                   ` (2 preceding siblings ...)
  2019-06-26  1:13 ` Song Liu
@ 2019-06-26 14:30 ` Daniel Borkmann
  3 siblings, 0 replies; 6+ messages in thread
From: Daniel Borkmann @ 2019-06-26 14:30 UTC (permalink / raw)
  To: Nathan Chancellor, Björn Töpel, Magnus Karlsson,
	David S. Miller, Alexei Starovoitov, Jakub Kicinski,
	Jesper Dangaard Brouer, John Fastabend
  Cc: netdev, bpf, xdp-newbies, linux-kernel, clang-built-linux,
	Nick Desaulniers, Nathan Huckleberry

On 06/25/2019 08:23 PM, Nathan Chancellor 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.
> 
> 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>

Applied, thanks!

^ permalink raw reply	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2019-06-26 14:30 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-06-25 18:23 [PATCH] xsk: Properly terminate assignment in xskq_produce_flush_desc Nathan Chancellor
2019-06-25 18:32 ` Nick Desaulniers
2019-06-25 19:46 ` Jonathan Lemon
2019-06-25 20:49   ` Björn Töpel
2019-06-26  1:13 ` Song Liu
2019-06-26 14:30 ` Daniel Borkmann

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