Re: [PATCH 10/11] x86, rwsem: provide __down_write_killable

[Borislav Petkov <bp@alien8.de>]

On Wed, Apr 20, 2016 at 11:04:05AM -0700, H. Peter Anvin wrote:
> The reason it breaks is because the same register can't be an
> input-output register and a separate input. However, the input side of
> the input-output is probably undefined, and so gcc may not notice.

So Michal and I talked about this a while ago. Why do we need the '"a"
(sem)' input dependency if '"+a" (ret)' already supplies the same thing?

There's also that "=d" (tmp) thing which we don't really need as an
output, right?

I.e., can we simplify like this?

---
#define ____down_write(sem, slow_path)                  \
({                                                      \
        long tmp = RWSEM_ACTIVE_WRITE_BIAS;             \
	struct rw_semaphore* ret = sem;			\
                                                        \
        asm volatile("# beginning down_write\n\t"       \
                     LOCK_PREFIX "  xadd      %[tmp],(%[ret])\n\t"      \
                     /* adds 0xffff0001, returns the old value */ \
                     "  test " __ASM_SEL(%w[tmp],%k[tmp]) "," __ASM_SEL(%w[tmp],%k[tmp]) "\n\t" \
                     /* was the active mask 0 before? */\
                     "  jz        1f\n"                 \
                     "  call " slow_path "\n"           \
                     "1:\n"                             \
                     "# ending down_write"              \
                     : "+m" (sem->count), [ret] "+a" (ret) \
                     : [tmp] "d" (tmp)                  \
                     : "memory", "cc");                 \
        ret;                                            \
})

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.