linux-kernel - Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait()

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Date:	Tue, 12 Apr 2016 17:59:41 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, waiman.long@....com,
	mingo@...hat.com, paulmck@...ux.vnet.ibm.com, boqun.feng@...il.com,
	torvalds@...ux-foundation.org, dave@...olabs.net
Subject: Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait()

Hi Peter,

Thanks for looking at this!

On Mon, Apr 04, 2016 at 02:22:53PM +0200, Peter Zijlstra wrote:
> Provide the cmpwait() primitive, which will 'spin' wait for a variable
> to change. Use it to implement smp_cond_load_acquire() and provide an
> ARM64 implementation.
> 
> The ARM64 implementation uses LDXR+WFE to avoid most spinning by
> letting the hardware go idle while waiting for the exclusive load of
> the variable to be cancelled (as anybody changing the value must).
> 
> I've misplaced my arm64 compiler, so this is not even compile tested.

Guess what? ;)

init/do_mounts_initrd.o: In function `__cmpwait_case_1':
/home/will/work/aarch32/linux/linux/./arch/arm64/include/asm/cmpxchg.h:242: multiple definition of `__cmpwait_case_1'
init/do_mounts.o:/home/will/work/aarch32/linux/linux/./arch/arm64/include/asm/cmpxchg.h:242: first defined here
init/do_mounts_initrd.o: In function `__cmpwait_case_2':
/home/will/work/aarch32/linux/linux/./arch/arm64/include/asm/cmpxchg.h:243: multiple definition of `__cmpwait_case_2'
init/do_mounts.o:/home/will/work/aarch32/linux/linux/./arch/arm64/include/asm/cmpxchg.h:243: first defined here
init/do_mounts_initrd.o: In function `__cmpwait_case_4':
/home/will/work/aarch32/linux/linux/./arch/arm64/include/asm/cmpxchg.h:244: multiple definition of `__cmpwait_case_4'
init/do_mounts.o:/home/will/work/aarch32/linux/linux/./arch/arm64/include/asm/cmpxchg.h:244: first defined here
init/do_mounts_initrd.o: In function `__cmpwait_case_8':
/home/will/work/aarch32/linux/linux/./arch/arm64/include/asm/cmpxchg.h:245: multiple definition of `__cmpwait_case_8'
init/do_mounts.o:/home/will/work/aarch32/linux/linux/./arch/arm64/include/asm/cmpxchg.h:245: first defined here
make[1]: *** [init/mounts.o] Error 1
make: *** [init] Error 2
make: *** Waiting for unfinished jobs....

(and lot of similar errors).

Looks like you're just missing an #undef in cmpxchg.h.

FWIW, you can pick up arm64 toolchain binaries from:

  https://releases.linaro.org/components/toolchain/binaries/latest-5/

> Suggested-by: Will Deacon <will.deacon@....com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/arm64/include/asm/cmpxchg.h |   36 +++++++++++++++++++++++++++++
>  include/linux/atomic.h           |   47 +++++++++++++++++++++++++++++++++++++++
>  include/linux/compiler.h         |   30 ------------------------
>  3 files changed, 83 insertions(+), 30 deletions(-)
> 
> --- a/arch/arm64/include/asm/cmpxchg.h
> +++ b/arch/arm64/include/asm/cmpxchg.h
> @@ -224,4 +224,40 @@ __CMPXCHG_GEN(_mb)
>  	__ret;								\
>  })
>  
> +#define __CMPWAIT_GEN(w, sz, name)					\
> +void __cmpwait_case_##name(volatile void *ptr, unsigned long val)	\
> +{									\
> +	unsigned long tmp;						\
> +									\
> +	asm volatile(							\
> +	"	ldxr" #sz "\t%" #w "[tmp], %[v]\n"			\
> +	"	eor	%" #w "[tmp], %" #w "[tmp], %" #w "[val]\n"	\
> +	"	cbnz	%" #w "[tmp], 1f\n"				\

Shouldn't this be cbz? (i.e. branch over the wfe if the value is equal
to what we wanted?).

> +	"	wfe\n"							\
> +	"1:"								\
> +	: [tmp] "=&r" (tmp), [val] "=&r" (val),				\

We only read val, so it can be an input operand, no?

> +	  [v] "+Q" (*(unsigned long *)ptr));				\
> +}
> +
> +__CMPWAIT_GEN(w, b, 1);
> +__CMPWAIT_GEN(w, h, 2);
> +__CMPWAIT_GEN(w,  , 4);
> +__CMPWAIT_GEN( ,  , 8);
> +
> +static inline void __cmpwait(volatile void *ptr, unsigned long val, int size)
> +{
> +	switch (size) {
> +	case 1: return __cmpwait_case_1(ptr, val);
> +	case 2: return __cmpwait_case_2(ptr, val);
> +	case 4: return __cmpwait_case_4(ptr, val);
> +	case 8: return __cmpwait_case_8(ptr, val);
> +	default: BUILD_BUG();
> +	}
> +
> +	unreachable();
> +}
> +
> +#define cmpwait(ptr, val) \
> +	__cmpwait((ptr), (unsigned long)(val), sizeof(*(ptr)))

We might want to call this cmpwait_relaxed, in case we decide to add
fenced versions in the future. Or just make it cmpwait_acquire and
remove the smp_rmb() from smp_cond_load_acquire(). Dunno.

Will