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

Michal Hocko
2016-04-17T16:59:21Z

On Wed 13-04-16 14:49:43, Michal Hocko wrote:
> On Wed 13-04-16 12:27:31, Ingo Molnar wrote:
> >
> > * Ingo Molnar <mingo@kernel.org> wrote:
> >
> > > I'm testing your patches today, if they are otherwise OK [...]
> >
> > got this build failure:
> >
> > ./arch/x86/include/asm/rwsem.h:106:2: error: ‘asm’ operand has impossible constraints
>
> Hmm, I have no idea why 64b didn't have problem with the asm but 32b
> complains. Anyway, the following makes both happy. I have checked the
> generated code for 64b and it hasn't changed after the patch. 32b also
> seems to be generating a proper code. My gcc asm()-foo is rather weak so
> I would feel better if somebody double checked after me.

Peter, Ingo, does the patch makes sense to you?
Thanks!

> ---
> From d23f4e6994670bf2c5d864f2190f21022b4499b2 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Wed, 13 Apr 2016 14:21:25 +0200
> Subject: [PATCH] x86: __down_read_trylock fix 32b build failure
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
>
> Ingo has noticed the following compilation error with CONFIG_X86_32=y
> ./arch/x86/include/asm/rwsem.h:106:2: error: ‘asm’ operand has impossible constraints
>
> The reason seems to be that 32b doesn't like ret being input and output
> argument sharing the same register with sem which is only the input. Fix
> this by making ret output only and use %3 (aka sem) for xadd.
>
> ret initialization is not needed now because this is done implicitly
> by the asm even for the fast path as both sem and ret share the same
> register.
>
> Reported-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> arch/x86/include/asm/rwsem.h | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
> index d759c5f70f49..453744c1d347 100644
> --- a/arch/x86/include/asm/rwsem.h
> +++ b/arch/x86/include/asm/rwsem.h
> @@ -102,9 +102,9 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
> #define ____down_write(sem, slow_path) \
> ({ \
> long tmp; \
> - struct rw_semaphore* ret = sem; \
> + struct rw_semaphore* ret; \
> asm volatile("# beginning down_write\n\t" \
> - LOCK_PREFIX " xadd %1,(%2)\n\t" \
> + LOCK_PREFIX " xadd %1,(%3)\n\t" \
> /* adds 0xffff0001, returns the old value */ \
> " test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
> /* was the active mask 0 before? */\
> @@ -112,7 +112,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
> " call " slow_path "\n" \
> "1:\n" \
> "# ending down_write" \
> - : "+m" (sem->count), "=d" (tmp), "+a" (ret) \
> + : "+m" (sem->count), "=d" (tmp), "=a" (ret) \
> : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
> : "memory", "cc"); \
> ret; \
> --
> 2.8.0.rc3
>
> --
> Michal Hocko
> SUSE Labs

--
Michal Hocko
SUSE Labs