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

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