Subject: Re: [RFC] arch: Introduce new TSO memory barrier smp_tmb()

From: "Paul E. McKenney" <[email-masked]>
Date: Mon, 4 Nov 2013 12:53:44 -0800

On Mon, Nov 04, 2013 at 08:11:27PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 04, 2013 at 08:27:32AM -0800, Paul E. McKenney wrote:
> > All this is leading me to suggest the following shortenings of names:
> > 
> > 	smp_load_with_acquire_semantics() -> smp_load_acquire()
> > 
> > 	smp_store_with_release_semantics() -> smp_store_release()
> > 
> > But names aside, the above gets rid of explicit barriers on TSO architectures,
> > allows ARM to avoid full DMB, and allows PowerPC to use lwsync instead of
> > the heavier-weight sync.
> 
> A little something like this? Completely guessed at the arm/arm64/ia64
> asm, but at least for those archs I found proper instructions (I hope),
> for x86,sparc,s390 which are TSO we can do with a barrier and PPC like
> said can do with the lwsync, all others fall back to using a smp_mb().
> 
> Should probably come with a proper changelog and an addition to _The_
> document.

Maybe something like this for the changelog?

	A number of situations currently require the heavyweight smp_mb(),
	even though there is no need to order prior stores against later
	loads.  Many architectures have much cheaper ways to handle these
	situations, but the Linux kernel currently has no portable way
	to make use of them.

	This commit therefore supplies smp_load_acquire() and
	smp_store_release() to remedy this situation.  The new
	smp_load_acquire() primitive orders the specified load against
	any subsequent reads or writes, while the new smp_store_release()
	primitive orders the specifed store against any prior reads or
	writes.  These primitives allow array-based circular FIFOs to be
	implemented without an smp_mb(), and also allow a theoretical
	hole in rcu_assign_pointer() to be closed at no additional
	expense on most architectures.

	In addition, the RCU experience transitioning from explicit
	smp_read_barrier_depends() and smp_wmb() to rcu_dereference()
	and rcu_assign_pointer(), respectively resulted in substantial
	improvements in readability.  It therefore seems likely that
	replacing other explicit barriers with smp_load_acquire() and
	smp_store_release() will provide similar benefits.  It appears
	that roughly half of the explicit barriers in core kernel code
	might be so replaced.

Some comments below.  I believe that opcodes need to be fixed for IA64.
I am unsure of the ifdefs and opcodes for arm64, but the ARM folks should
be able to tell us.

Other than that, for the rest:

Reviewed-by: Paul E. McKenney <[email-masked]>

> ---
>  arch/alpha/include/asm/barrier.h      | 13 +++++++++++
>  arch/arc/include/asm/barrier.h        | 13 +++++++++++
>  arch/arm/include/asm/barrier.h        | 26 +++++++++++++++++++++
>  arch/arm64/include/asm/barrier.h      | 28 +++++++++++++++++++++++
>  arch/avr32/include/asm/barrier.h      | 12 ++++++++++
>  arch/blackfin/include/asm/barrier.h   | 13 +++++++++++
>  arch/cris/include/asm/barrier.h       | 13 +++++++++++
>  arch/frv/include/asm/barrier.h        | 13 +++++++++++
>  arch/h8300/include/asm/barrier.h      | 13 +++++++++++
>  arch/hexagon/include/asm/barrier.h    | 13 +++++++++++
>  arch/ia64/include/asm/barrier.h       | 43 +++++++++++++++++++++++++++++++++++
>  arch/m32r/include/asm/barrier.h       | 13 +++++++++++
>  arch/m68k/include/asm/barrier.h       | 13 +++++++++++
>  arch/metag/include/asm/barrier.h      | 13 +++++++++++
>  arch/microblaze/include/asm/barrier.h | 13 +++++++++++
>  arch/mips/include/asm/barrier.h       | 13 +++++++++++
>  arch/mn10300/include/asm/barrier.h    | 13 +++++++++++
>  arch/parisc/include/asm/barrier.h     | 13 +++++++++++
>  arch/powerpc/include/asm/barrier.h    | 15 ++++++++++++
>  arch/s390/include/asm/barrier.h       | 13 +++++++++++
>  arch/score/include/asm/barrier.h      | 13 +++++++++++
>  arch/sh/include/asm/barrier.h         | 13 +++++++++++
>  arch/sparc/include/asm/barrier_32.h   | 13 +++++++++++
>  arch/sparc/include/asm/barrier_64.h   | 13 +++++++++++
>  arch/tile/include/asm/barrier.h       | 13 +++++++++++
>  arch/unicore32/include/asm/barrier.h  | 13 +++++++++++
>  arch/x86/include/asm/barrier.h        | 13 +++++++++++
>  arch/xtensa/include/asm/barrier.h     | 13 +++++++++++
>  28 files changed, 423 insertions(+)
> 
> diff --git a/arch/alpha/include/asm/barrier.h b/arch/alpha/include/asm/barrier.h
> index ce8860a0b32d..464139feee97 100644
> --- a/arch/alpha/include/asm/barrier.h
> +++ b/arch/alpha/include/asm/barrier.h
> @@ -29,6 +29,19 @@ __asm__ __volatile__("mb": : :"memory")
>  #define smp_read_barrier_depends()	do { } while (0)
>  #endif
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Yep, not any alternative to smp_mb() here.

>  #define set_mb(var, value) \
>  do { var = value; mb(); } while (0)
> 
> diff --git a/arch/arc/include/asm/barrier.h b/arch/arc/include/asm/barrier.h
> index f6cb7c4ffb35..a779da846fb5 100644
> --- a/arch/arc/include/asm/barrier.h
> +++ b/arch/arc/include/asm/barrier.h
> @@ -30,6 +30,19 @@
>  #define smp_wmb()       barrier()
>  #endif
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Appears to be !SMP, so OK.

>  #define smp_mb__before_atomic_dec()	barrier()
>  #define smp_mb__after_atomic_dec()	barrier()
>  #define smp_mb__before_atomic_inc()	barrier()
> diff --git a/arch/arm/include/asm/barrier.h b/arch/arm/include/asm/barrier.h
> index 60f15e274e6d..a804093d6891 100644
> --- a/arch/arm/include/asm/barrier.h
> +++ b/arch/arm/include/asm/barrier.h
> @@ -53,10 +53,36 @@
>  #define smp_mb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
> +
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
>  #else
>  #define smp_mb()	dmb(ish)
>  #define smp_rmb()	smp_mb()
>  #define smp_wmb()	dmb(ishst)
> +

Seems like there should be some sort of #ifdef condition to distinguish
between these.  My guess is something like:

#if __LINUX_ARM_ARCH__ > 7

But I must defer to the ARM guys.  For all I know, they might prefer
arch/arm to stick with smp_mb() and have arch/arm64 do the ldar and stlr.

> +#define smp_store_release(p, v)						\
> +do {									\
> +	asm volatile ("stlr %w0 [%1]" : : "r" (v), "r" (&p) : "memory");\
> +} while (0)
> +
> +#define smp_load_acquire(p)						\
> +do {									\
> +	typeof(p) ___p1;						\
> +	asm volatile ("ldar %w0, [%1]"					\
> +			: "=r" (___p1) : "r" (&p) : "memory");		\
> +	return ___p1;							\
> +} while (0)
>  #endif
> 
>  #define read_barrier_depends()		do { } while(0)
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index d4a63338a53c..0da2d4ebb9a8 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -35,10 +35,38 @@
>  #define smp_mb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
> +
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +
>  #else
> +
>  #define smp_mb()	asm volatile("dmb ish" : : : "memory")
>  #define smp_rmb()	asm volatile("dmb ishld" : : : "memory")
>  #define smp_wmb()	asm volatile("dmb ishst" : : : "memory")
> +
> +#define smp_store_release(p, v)						\
> +do {									\
> +	asm volatile ("stlr %w0 [%1]" : : "r" (v), "r" (&p) : "memory");\
> +} while (0)
> +
> +#define smp_load_acquire(p)						\
> +do {									\
> +	typeof(p) ___p1;						\
> +	asm volatile ("ldar %w0, [%1]"					\
> +			: "=r" (___p1) : "r" (&p) : "memory");		\
> +	return ___p1;							\
> +} while (0)
>  #endif

Ditto on the instruction format.  The closest thing I see in the kernel
is "stlr %w1, %0" in arch_write_unlock() and arch_spin_unlock().

> 
>  #define read_barrier_depends()		do { } while(0)
> diff --git a/arch/avr32/include/asm/barrier.h b/arch/avr32/include/asm/barrier.h
> index 0961275373db..a0c48ad684f8 100644
> --- a/arch/avr32/include/asm/barrier.h
> +++ b/arch/avr32/include/asm/barrier.h
> @@ -25,5 +25,17 @@
>  # define smp_read_barrier_depends() do { } while(0)
>  #endif
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)

!SMP, so should be OK.

> 
>  #endif /* __ASM_AVR32_BARRIER_H */
> diff --git a/arch/blackfin/include/asm/barrier.h b/arch/blackfin/include/asm/barrier.h
> index ebb189507dd7..67889d9225d9 100644
> --- a/arch/blackfin/include/asm/barrier.h
> +++ b/arch/blackfin/include/asm/barrier.h
> @@ -45,4 +45,17 @@
>  #define set_mb(var, value) do { var = value; mb(); } while (0)
>  #define smp_read_barrier_depends()	read_barrier_depends()
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Ditto.

>  #endif /* _BLACKFIN_BARRIER_H */
> diff --git a/arch/cris/include/asm/barrier.h b/arch/cris/include/asm/barrier.h
> index 198ad7fa6b25..34243dc44ef1 100644
> --- a/arch/cris/include/asm/barrier.h
> +++ b/arch/cris/include/asm/barrier.h
> @@ -22,4 +22,17 @@
>  #define smp_read_barrier_depends()     do { } while(0)
>  #endif
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Ditto.

>  #endif /* __ASM_CRIS_BARRIER_H */
> diff --git a/arch/frv/include/asm/barrier.h b/arch/frv/include/asm/barrier.h
> index 06776ad9f5e9..92f89934d4ed 100644
> --- a/arch/frv/include/asm/barrier.h
> +++ b/arch/frv/include/asm/barrier.h
> @@ -26,4 +26,17 @@
>  #define set_mb(var, value) \
>  	do { var = (value); barrier(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Ditto.

>  #endif /* _ASM_BARRIER_H */
> diff --git a/arch/h8300/include/asm/barrier.h b/arch/h8300/include/asm/barrier.h
> index 9e0aa9fc195d..516e9d379e25 100644
> --- a/arch/h8300/include/asm/barrier.h
> +++ b/arch/h8300/include/asm/barrier.h
> @@ -26,4 +26,17 @@
>  #define smp_read_barrier_depends()	do { } while(0)
>  #endif
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

And ditto again...

>  #endif /* _H8300_BARRIER_H */
> diff --git a/arch/hexagon/include/asm/barrier.h b/arch/hexagon/include/asm/barrier.h
> index 1041a8e70ce8..838a2ebe07a5 100644
> --- a/arch/hexagon/include/asm/barrier.h
> +++ b/arch/hexagon/include/asm/barrier.h
> @@ -38,4 +38,17 @@
>  #define set_mb(var, value) \
>  	do { var = value; mb(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

And again...

>  #endif /* _ASM_BARRIER_H */
> diff --git a/arch/ia64/include/asm/barrier.h b/arch/ia64/include/asm/barrier.h
> index 60576e06b6fb..4598d390fabb 100644
> --- a/arch/ia64/include/asm/barrier.h
> +++ b/arch/ia64/include/asm/barrier.h
> @@ -45,11 +45,54 @@
>  # define smp_rmb()	rmb()
>  # define smp_wmb()	wmb()
>  # define smp_read_barrier_depends()	read_barrier_depends()
> +
> +#define smp_store_release(p, v)						\
> +do {									\
> +	switch (sizeof(p)) {						\
> +	case 4:								\
> +		asm volatile ("st4.acq [%0]=%1" 			\

This should be "st4.rel".

> +				:: "r" (&p), "r" (v) : "memory");	\
> +		break;							\
> +	case 8:								\
> +		asm volatile ("st8.acq [%0]=%1" 			\

And this should be "st8.rel"

> +				:: "r" (&p), "r" (v) : "memory"); 	\
> +		break;							\
> +	}								\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1;						\
> +	switch (sizeof(p)) {						\
> +	case 4:								\
> +		asm volatile ("ld4.rel %0=[%1]" 			\

And this should be "ld4.acq".

> +				: "=r"(___p1) : "r" (&p) : "memory"); 	\
> +		break;							\
> +	case 8:								\
> +		asm volatile ("ld8.rel %0=[%1]" 			\

And this should be "ld8.acq".

> +				: "=r"(___p1) : "r" (&p) : "memory"); 	\
> +		break;							\
> +	}								\
> +	return ___p1;							\
> +} while (0)

It appears that sizes 2 and 1 are also available, but 4 and 8 seem like
good places to start.

>  #else
>  # define smp_mb()	barrier()
>  # define smp_rmb()	barrier()
>  # define smp_wmb()	barrier()
>  # define smp_read_barrier_depends()	do { } while(0)
> +
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
>  #endif
> 
>  /*
> diff --git a/arch/m32r/include/asm/barrier.h b/arch/m32r/include/asm/barrier.h
> index 6976621efd3f..e5d42bcf90c5 100644
> --- a/arch/m32r/include/asm/barrier.h
> +++ b/arch/m32r/include/asm/barrier.h
> @@ -91,4 +91,17 @@
>  #define set_mb(var, value) do { var = value; barrier(); } while (0)
>  #endif
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Another !SMP architecture, so looks good.

>  #endif /* _ASM_M32R_BARRIER_H */
> diff --git a/arch/m68k/include/asm/barrier.h b/arch/m68k/include/asm/barrier.h
> index 445ce22c23cb..eeb9ecf713cc 100644
> --- a/arch/m68k/include/asm/barrier.h
> +++ b/arch/m68k/include/asm/barrier.h
> @@ -17,4 +17,17 @@
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	((void)0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Ditto.

>  #endif /* _M68K_BARRIER_H */
> diff --git a/arch/metag/include/asm/barrier.h b/arch/metag/include/asm/barrier.h
> index c90bfc6bf648..d8e6f2e4a27c 100644
> --- a/arch/metag/include/asm/barrier.h
> +++ b/arch/metag/include/asm/barrier.h
> @@ -82,4 +82,17 @@ static inline void fence(void)
>  #define smp_read_barrier_depends()     do { } while (0)
>  #define set_mb(var, value) do { var = value; smp_mb(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

This one is a bit unusual, but use of smp_mb() should be safe.

>  #endif /* _ASM_METAG_BARRIER_H */
> diff --git a/arch/microblaze/include/asm/barrier.h b/arch/microblaze/include/asm/barrier.h
> index df5be3e87044..a890702061c9 100644
> --- a/arch/microblaze/include/asm/barrier.h
> +++ b/arch/microblaze/include/asm/barrier.h
> @@ -24,4 +24,17 @@
>  #define smp_rmb()		rmb()
>  #define smp_wmb()		wmb()
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

!SMP only, so good.

>  #endif /* _ASM_MICROBLAZE_BARRIER_H */
> diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
> index 314ab5532019..e59bcd051f36 100644
> --- a/arch/mips/include/asm/barrier.h
> +++ b/arch/mips/include/asm/barrier.h
> @@ -180,4 +180,17 @@
>  #define nudge_writes() mb()
>  #endif
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Interesting variety here as well.  Again, smp_mb() should be safe.

>  #endif /* __ASM_BARRIER_H */
> diff --git a/arch/mn10300/include/asm/barrier.h b/arch/mn10300/include/asm/barrier.h
> index 2bd97a5c8af7..0e6a0608d4a1 100644
> --- a/arch/mn10300/include/asm/barrier.h
> +++ b/arch/mn10300/include/asm/barrier.h
> @@ -34,4 +34,17 @@
>  #define read_barrier_depends()		do {} while (0)
>  #define smp_read_barrier_depends()	do {} while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

!SMP, so good.

>  #endif /* _ASM_BARRIER_H */
> diff --git a/arch/parisc/include/asm/barrier.h b/arch/parisc/include/asm/barrier.h
> index e77d834aa803..f1145a8594a0 100644
> --- a/arch/parisc/include/asm/barrier.h
> +++ b/arch/parisc/include/asm/barrier.h
> @@ -32,4 +32,17 @@
> 
>  #define set_mb(var, value)		do { var = value; mb(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Ditto.

>  #endif /* __PARISC_BARRIER_H */
> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index ae782254e731..b5cc36791f42 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -65,4 +65,19 @@
>  #define data_barrier(x)	\
>  	asm volatile("twi 0,%0,0; isync" : : "r" (x) : "memory");
> 
> +/* use smp_rmb() as that is either lwsync or a barrier() depending on SMP */
> +
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_rmb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_rmb();							\
> +	return ___p1;							\
> +} while (0)
> +

I think that this actually does work, strange though it does look.

>  #endif /* _ASM_POWERPC_BARRIER_H */
> diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h
> index 16760eeb79b0..e8989c40e11c 100644
> --- a/arch/s390/include/asm/barrier.h
> +++ b/arch/s390/include/asm/barrier.h
> @@ -32,4 +32,17 @@
> 
>  #define set_mb(var, value)		do { var = value; mb(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	barrier();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	barrier();							\
> +	return ___p1;							\
> +} while (0)
> +

I believe that this is OK as well, but must defer to the s390
maintainers.

>  #endif /* __ASM_BARRIER_H */
> diff --git a/arch/score/include/asm/barrier.h b/arch/score/include/asm/barrier.h
> index 0eacb6471e6d..5f101ef8ade9 100644
> --- a/arch/score/include/asm/barrier.h
> +++ b/arch/score/include/asm/barrier.h
> @@ -13,4 +13,17 @@
> 
>  #define set_mb(var, value) 		do {var = value; wmb(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

!SMP, so good.

>  #endif /* _ASM_SCORE_BARRIER_H */
> diff --git a/arch/sh/include/asm/barrier.h b/arch/sh/include/asm/barrier.h
> index 72c103dae300..611128c2f636 100644
> --- a/arch/sh/include/asm/barrier.h
> +++ b/arch/sh/include/asm/barrier.h
> @@ -51,4 +51,17 @@
> 
>  #define set_mb(var, value) do { (void)xchg(&var, value); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

Use of smp_mb() should be safe here.

>  #endif /* __ASM_SH_BARRIER_H */
> diff --git a/arch/sparc/include/asm/barrier_32.h b/arch/sparc/include/asm/barrier_32.h
> index c1b76654ee76..f47f9d51f326 100644
> --- a/arch/sparc/include/asm/barrier_32.h
> +++ b/arch/sparc/include/asm/barrier_32.h
> @@ -12,4 +12,17 @@
>  #define smp_wmb()	__asm__ __volatile__("":::"memory")
>  #define smp_read_barrier_depends()	do { } while(0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

The surrounding code looks to be set up for !SMP.  I -thought- that there
were SMP 32-bit SPARC systems, but either way, smp_mb() should be safe.

>  #endif /* !(__SPARC_BARRIER_H) */
> diff --git a/arch/sparc/include/asm/barrier_64.h b/arch/sparc/include/asm/barrier_64.h
> index 95d45986f908..77cbe6982ca0 100644
> --- a/arch/sparc/include/asm/barrier_64.h
> +++ b/arch/sparc/include/asm/barrier_64.h
> @@ -53,4 +53,17 @@ do {	__asm__ __volatile__("ba,pt	%%xcc, 1f\n\t" \
> 
>  #define smp_read_barrier_depends()	do { } while(0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	barrier();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	barrier();							\
> +	return ___p1;							\
> +} while (0)
> +

SPARC64 is TSO, so looks good.

>  #endif /* !(__SPARC64_BARRIER_H) */
> diff --git a/arch/tile/include/asm/barrier.h b/arch/tile/include/asm/barrier.h
> index a9a73da5865d..4d5330d4fd31 100644
> --- a/arch/tile/include/asm/barrier.h
> +++ b/arch/tile/include/asm/barrier.h
> @@ -140,5 +140,18 @@ mb_incoherent(void)
>  #define set_mb(var, value) \
>  	do { var = value; mb(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

The __mb_incoherent() in the surrounding code looks scary, but smp_mb()
should suffice here as well as elsewhere.

>  #endif /* !__ASSEMBLY__ */
>  #endif /* _ASM_TILE_BARRIER_H */
> diff --git a/arch/unicore32/include/asm/barrier.h b/arch/unicore32/include/asm/barrier.h
> index a6620e5336b6..5471ff6aae10 100644
> --- a/arch/unicore32/include/asm/barrier.h
> +++ b/arch/unicore32/include/asm/barrier.h
> @@ -25,4 +25,17 @@
> 
>  #define set_mb(var, value)		do { var = value; smp_mb(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

!SMP, so good.

>  #endif /* __UNICORE_BARRIER_H__ */
> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
> index c6cd358a1eec..a7fd8201ab09 100644
> --- a/arch/x86/include/asm/barrier.h
> +++ b/arch/x86/include/asm/barrier.h
> @@ -100,6 +100,19 @@
>  #define set_mb(var, value) do { var = value; barrier(); } while (0)
>  #endif
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	barrier();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	barrier();							\
> +	return ___p1;							\
> +} while (0)
> +

TSO, so good.

>  /*
>   * Stop RDTSC speculation. This is needed when you need to use RDTSC
>   * (or get_cycles or vread that possibly accesses the TSC) in a defined
> diff --git a/arch/xtensa/include/asm/barrier.h b/arch/xtensa/include/asm/barrier.h
> index ef021677d536..703d511add49 100644
> --- a/arch/xtensa/include/asm/barrier.h
> +++ b/arch/xtensa/include/asm/barrier.h
> @@ -26,4 +26,17 @@
> 
>  #define set_mb(var, value)	do { var = value; mb(); } while (0)
> 
> +#define smp_store_release(p, v)						\
> +do {									\
> +	smp_mb();							\
> +	ACCESS_ONCE(p) = (v);						\
> +} while (0)
> +
> +#define smp_load_acquire(p, v)						\
> +do {									\
> +	typeof(p) ___p1 = ACCESS_ONCE(p);				\
> +	smp_mb();							\
> +	return ___p1;							\
> +} while (0)
> +

The use of smp_mb() should be safe, so good.  Looks like xtensa orders
reads, but not writes -- interesting...

>  #endif /* _XTENSA_SYSTEM_H */
>