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

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 04 Nov 2013 07:59:23 +1100

On Sun, 2013-11-03 at 16:17 +0100, Peter Zijlstra wrote:
> On Sun, Nov 03, 2013 at 06:40:17AM -0800, Paul E. McKenney wrote:
> > If there was an smp_tmb(), I would likely use it in rcu_assign_pointer().
> 
> Well, I'm obviously all for introducing this new barrier, for it will
> reduce a full mfence on x86 to a compiler barrier. And ppc can use
> lwsync as opposed to sync afaict. Not sure ARM can do better.

The patch at the *very least* needs a good description of the semantics
of the barrier, what does it order vs. what etc...

Cheers,
Ben.

> ---
> Subject: arch: Introduce new TSO memory barrier smp_tmb()
> 
> A few sites could be downgraded from smp_mb() to smp_tmb() and a few
> site should be upgraded to smp_tmb() that are now using smp_wmb().
> 
> XXX hope PaulMck explains things better..
> 
> X86 (!OOSTORE), SPARC have native TSO memory models and smp_tmb()
> reduces to barrier().
> 
> PPC can use lwsync instead of sync
> 
> For the other archs, have smp_tmb map to smp_mb, as the stronger barrier
> is always correct but possibly suboptimal.
> 
> Suggested-by: Paul McKenney <[email-masked]>
> Not-Signed-off-by: Peter Zijlstra <[email-masked]>
> ---
>  arch/alpha/include/asm/barrier.h      | 2 ++
>  arch/arc/include/asm/barrier.h        | 2 ++
>  arch/arm/include/asm/barrier.h        | 2 ++
>  arch/arm64/include/asm/barrier.h      | 2 ++
>  arch/avr32/include/asm/barrier.h      | 1 +
>  arch/blackfin/include/asm/barrier.h   | 1 +
>  arch/cris/include/asm/barrier.h       | 2 ++
>  arch/frv/include/asm/barrier.h        | 1 +
>  arch/h8300/include/asm/barrier.h      | 2 ++
>  arch/hexagon/include/asm/barrier.h    | 1 +
>  arch/ia64/include/asm/barrier.h       | 2 ++
>  arch/m32r/include/asm/barrier.h       | 2 ++
>  arch/m68k/include/asm/barrier.h       | 1 +
>  arch/metag/include/asm/barrier.h      | 3 +++
>  arch/microblaze/include/asm/barrier.h | 1 +
>  arch/mips/include/asm/barrier.h       | 3 +++
>  arch/mn10300/include/asm/barrier.h    | 2 ++
>  arch/parisc/include/asm/barrier.h     | 1 +
>  arch/powerpc/include/asm/barrier.h    | 2 ++
>  arch/s390/include/asm/barrier.h       | 1 +
>  arch/score/include/asm/barrier.h      | 1 +
>  arch/sh/include/asm/barrier.h         | 2 ++
>  arch/sparc/include/asm/barrier_32.h   | 1 +
>  arch/sparc/include/asm/barrier_64.h   | 3 +++
>  arch/tile/include/asm/barrier.h       | 2 ++
>  arch/unicore32/include/asm/barrier.h  | 1 +
>  arch/x86/include/asm/barrier.h        | 3 +++
>  arch/xtensa/include/asm/barrier.h     | 1 +
>  28 files changed, 48 insertions(+)
> 
> diff --git a/arch/alpha/include/asm/barrier.h b/arch/alpha/include/asm/barrier.h
> index ce8860a0b32d..02ea63897038 100644
> --- a/arch/alpha/include/asm/barrier.h
> +++ b/arch/alpha/include/asm/barrier.h
> @@ -18,12 +18,14 @@ __asm__ __volatile__("mb": : :"memory")
>  #ifdef CONFIG_SMP
>  #define __ASM_SMP_MB	"\tmb\n"
>  #define smp_mb()	mb()
> +#define smp_tmb()	mb()
>  #define smp_rmb()	rmb()
>  #define smp_wmb()	wmb()
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #else
>  #define __ASM_SMP_MB
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	do { } while (0)
> diff --git a/arch/arc/include/asm/barrier.h b/arch/arc/include/asm/barrier.h
> index f6cb7c4ffb35..456c790fa1ad 100644
> --- a/arch/arc/include/asm/barrier.h
> +++ b/arch/arc/include/asm/barrier.h
> @@ -22,10 +22,12 @@
>  /* TODO-vineetg verify the correctness of macros here */
>  #ifdef CONFIG_SMP
>  #define smp_mb()        mb()
> +#define smp_tmb()	mb()
>  #define smp_rmb()       rmb()
>  #define smp_wmb()       wmb()
>  #else
>  #define smp_mb()        barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()       barrier()
>  #define smp_wmb()       barrier()
>  #endif
> diff --git a/arch/arm/include/asm/barrier.h b/arch/arm/include/asm/barrier.h
> index 60f15e274e6d..bc88a8505673 100644
> --- a/arch/arm/include/asm/barrier.h
> +++ b/arch/arm/include/asm/barrier.h
> @@ -51,10 +51,12 @@
>  
>  #ifndef CONFIG_SMP
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #else
>  #define smp_mb()	dmb(ish)
> +#define smp_tmb()	smp_mb()
>  #define smp_rmb()	smp_mb()
>  #define smp_wmb()	dmb(ishst)
>  #endif
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index d4a63338a53c..ec0531f4892f 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -33,10 +33,12 @@
>  
>  #ifndef CONFIG_SMP
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #else
>  #define smp_mb()	asm volatile("dmb ish" : : : "memory")
> +#define smp_tmb()	asm volatile("dmb ish" : : : "memory")
>  #define smp_rmb()	asm volatile("dmb ishld" : : : "memory")
>  #define smp_wmb()	asm volatile("dmb ishst" : : : "memory")
>  #endif
> diff --git a/arch/avr32/include/asm/barrier.h b/arch/avr32/include/asm/barrier.h
> index 0961275373db..6c6ccb9cf290 100644
> --- a/arch/avr32/include/asm/barrier.h
> +++ b/arch/avr32/include/asm/barrier.h
> @@ -20,6 +20,7 @@
>  # error "The AVR32 port does not support SMP"
>  #else
>  # define smp_mb()		barrier()
> +# define smp_tmb()		barrier()
>  # define smp_rmb()		barrier()
>  # define smp_wmb()		barrier()
>  # define smp_read_barrier_depends() do { } while(0)
> diff --git a/arch/blackfin/include/asm/barrier.h b/arch/blackfin/include/asm/barrier.h
> index ebb189507dd7..100f49121a18 100644
> --- a/arch/blackfin/include/asm/barrier.h
> +++ b/arch/blackfin/include/asm/barrier.h
> @@ -40,6 +40,7 @@
>  #endif /* !CONFIG_SMP */
>  
>  #define smp_mb()  mb()
> +#define smp_tmb() mb()
>  #define smp_rmb() rmb()
>  #define smp_wmb() wmb()
>  #define set_mb(var, value) do { var = value; mb(); } while (0)
> diff --git a/arch/cris/include/asm/barrier.h b/arch/cris/include/asm/barrier.h
> index 198ad7fa6b25..679c33738b4c 100644
> --- a/arch/cris/include/asm/barrier.h
> +++ b/arch/cris/include/asm/barrier.h
> @@ -12,11 +12,13 @@
>  
>  #ifdef CONFIG_SMP
>  #define smp_mb()        mb()
> +#define smp_tmb()       mb()
>  #define smp_rmb()       rmb()
>  #define smp_wmb()       wmb()
>  #define smp_read_barrier_depends()     read_barrier_depends()
>  #else
>  #define smp_mb()        barrier()
> +#define smp_tmb()       barrier()
>  #define smp_rmb()       barrier()
>  #define smp_wmb()       barrier()
>  #define smp_read_barrier_depends()     do { } while(0)
> diff --git a/arch/frv/include/asm/barrier.h b/arch/frv/include/asm/barrier.h
> index 06776ad9f5e9..60354ce13ba0 100644
> --- a/arch/frv/include/asm/barrier.h
> +++ b/arch/frv/include/asm/barrier.h
> @@ -20,6 +20,7 @@
>  #define read_barrier_depends()	do { } while (0)
>  
>  #define smp_mb()			barrier()
> +#define smp_tmb()			barrier()
>  #define smp_rmb()			barrier()
>  #define smp_wmb()			barrier()
>  #define smp_read_barrier_depends()	do {} while(0)
> diff --git a/arch/h8300/include/asm/barrier.h b/arch/h8300/include/asm/barrier.h
> index 9e0aa9fc195d..e8e297fa4e9a 100644
> --- a/arch/h8300/include/asm/barrier.h
> +++ b/arch/h8300/include/asm/barrier.h
> @@ -16,11 +16,13 @@
>  
>  #ifdef CONFIG_SMP
>  #define smp_mb()	mb()
> +#define smp_tmb()	mb()
>  #define smp_rmb()	rmb()
>  #define smp_wmb()	wmb()
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #else
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	do { } while(0)
> diff --git a/arch/hexagon/include/asm/barrier.h b/arch/hexagon/include/asm/barrier.h
> index 1041a8e70ce8..2dd5b2ad4d21 100644
> --- a/arch/hexagon/include/asm/barrier.h
> +++ b/arch/hexagon/include/asm/barrier.h
> @@ -28,6 +28,7 @@
>  #define smp_rmb()			barrier()
>  #define smp_read_barrier_depends()	barrier()
>  #define smp_wmb()			barrier()
> +#define smp_tmb()			barrier()
>  #define smp_mb()			barrier()
>  #define smp_mb__before_atomic_dec()	barrier()
>  #define smp_mb__after_atomic_dec()	barrier()
> diff --git a/arch/ia64/include/asm/barrier.h b/arch/ia64/include/asm/barrier.h
> index 60576e06b6fb..a5f92146b091 100644
> --- a/arch/ia64/include/asm/barrier.h
> +++ b/arch/ia64/include/asm/barrier.h
> @@ -42,11 +42,13 @@
>  
>  #ifdef CONFIG_SMP
>  # define smp_mb()	mb()
> +# define smp_tmb()	mb()
>  # define smp_rmb()	rmb()
>  # define smp_wmb()	wmb()
>  # define smp_read_barrier_depends()	read_barrier_depends()
>  #else
>  # define smp_mb()	barrier()
> +# define smp_tmb()	barrier()
>  # define smp_rmb()	barrier()
>  # define smp_wmb()	barrier()
>  # define smp_read_barrier_depends()	do { } while(0)
> diff --git a/arch/m32r/include/asm/barrier.h b/arch/m32r/include/asm/barrier.h
> index 6976621efd3f..a6fa29facd7a 100644
> --- a/arch/m32r/include/asm/barrier.h
> +++ b/arch/m32r/include/asm/barrier.h
> @@ -79,12 +79,14 @@
>  
>  #ifdef CONFIG_SMP
>  #define smp_mb()	mb()
> +#define smp_tmb()	mb()
>  #define smp_rmb()	rmb()
>  #define smp_wmb()	wmb()
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #define set_mb(var, value) do { (void) xchg(&var, value); } while (0)
>  #else
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	do { } while (0)
> diff --git a/arch/m68k/include/asm/barrier.h b/arch/m68k/include/asm/barrier.h
> index 445ce22c23cb..8ecf52c87847 100644
> --- a/arch/m68k/include/asm/barrier.h
> +++ b/arch/m68k/include/asm/barrier.h
> @@ -13,6 +13,7 @@
>  #define set_mb(var, value)	({ (var) = (value); wmb(); })
>  
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	((void)0)
> diff --git a/arch/metag/include/asm/barrier.h b/arch/metag/include/asm/barrier.h
> index c90bfc6bf648..eb179fbce580 100644
> --- a/arch/metag/include/asm/barrier.h
> +++ b/arch/metag/include/asm/barrier.h
> @@ -50,6 +50,7 @@ static inline void wmb(void)
>  #ifndef CONFIG_SMP
>  #define fence()		do { } while (0)
>  #define smp_mb()        barrier()
> +#define smp_tmb()       barrier()
>  #define smp_rmb()       barrier()
>  #define smp_wmb()       barrier()
>  #else
> @@ -70,11 +71,13 @@ static inline void fence(void)
>  	*flushptr = 0;
>  }
>  #define smp_mb()        fence()
> +#define smp_tmb()       fence()
>  #define smp_rmb()       fence()
>  #define smp_wmb()       barrier()
>  #else
>  #define fence()		do { } while (0)
>  #define smp_mb()        barrier()
> +#define smp_tmb()       barrier()
>  #define smp_rmb()       barrier()
>  #define smp_wmb()       barrier()
>  #endif
> diff --git a/arch/microblaze/include/asm/barrier.h b/arch/microblaze/include/asm/barrier.h
> index df5be3e87044..d573c170a717 100644
> --- a/arch/microblaze/include/asm/barrier.h
> +++ b/arch/microblaze/include/asm/barrier.h
> @@ -21,6 +21,7 @@
>  #define set_wmb(var, value)	do { var = value; wmb(); } while (0)
>  
>  #define smp_mb()		mb()
> +#define smp_tmb()		mb()
>  #define smp_rmb()		rmb()
>  #define smp_wmb()		wmb()
>  
> diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
> index 314ab5532019..535e699eec3b 100644
> --- a/arch/mips/include/asm/barrier.h
> +++ b/arch/mips/include/asm/barrier.h
> @@ -144,15 +144,18 @@
>  #if defined(CONFIG_WEAK_ORDERING) && defined(CONFIG_SMP)
>  # ifdef CONFIG_CPU_CAVIUM_OCTEON
>  #  define smp_mb()	__sync()
> +#  define smp_tmb()	__sync()
>  #  define smp_rmb()	barrier()
>  #  define smp_wmb()	__syncw()
>  # else
>  #  define smp_mb()	__asm__ __volatile__("sync" : : :"memory")
> +#  define smp_tmb()	__asm__ __volatile__("sync" : : :"memory")
>  #  define smp_rmb()	__asm__ __volatile__("sync" : : :"memory")
>  #  define smp_wmb()	__asm__ __volatile__("sync" : : :"memory")
>  # endif
>  #else
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #endif
> diff --git a/arch/mn10300/include/asm/barrier.h b/arch/mn10300/include/asm/barrier.h
> index 2bd97a5c8af7..a345b0776e5f 100644
> --- a/arch/mn10300/include/asm/barrier.h
> +++ b/arch/mn10300/include/asm/barrier.h
> @@ -19,11 +19,13 @@
>  
>  #ifdef CONFIG_SMP
>  #define smp_mb()	mb()
> +#define smp_tmb()	mb()
>  #define smp_rmb()	rmb()
>  #define smp_wmb()	wmb()
>  #define set_mb(var, value)  do { xchg(&var, value); } while (0)
>  #else  /* CONFIG_SMP */
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define set_mb(var, value)  do { var = value;  mb(); } while (0)
> diff --git a/arch/parisc/include/asm/barrier.h b/arch/parisc/include/asm/barrier.h
> index e77d834aa803..f53196b589ec 100644
> --- a/arch/parisc/include/asm/barrier.h
> +++ b/arch/parisc/include/asm/barrier.h
> @@ -25,6 +25,7 @@
>  #define rmb()		mb()
>  #define wmb()		mb()
>  #define smp_mb()	mb()
> +#define smp_tmb()	mb()
>  #define smp_rmb()	mb()
>  #define smp_wmb()	mb()
>  #define smp_read_barrier_depends()	do { } while(0)
> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index ae782254e731..d7e8a560f1fe 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -46,11 +46,13 @@
>  #endif
>  
>  #define smp_mb()	mb()
> +#define smp_tmb()	__asm__ __volatile__ (stringify_in_c(LWSYNC) : : :"memory")
>  #define smp_rmb()	__asm__ __volatile__ (stringify_in_c(LWSYNC) : : :"memory")
>  #define smp_wmb()	__asm__ __volatile__ (stringify_in_c(SMPWMB) : : :"memory")
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #else
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	do { } while(0)
> diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h
> index 16760eeb79b0..f0409a874243 100644
> --- a/arch/s390/include/asm/barrier.h
> +++ b/arch/s390/include/asm/barrier.h
> @@ -24,6 +24,7 @@
>  #define wmb()				mb()
>  #define read_barrier_depends()		do { } while(0)
>  #define smp_mb()			mb()
> +#define smp_tmb()			mb()
>  #define smp_rmb()			rmb()
>  #define smp_wmb()			wmb()
>  #define smp_read_barrier_depends()	read_barrier_depends()
> diff --git a/arch/score/include/asm/barrier.h b/arch/score/include/asm/barrier.h
> index 0eacb6471e6d..865652083dde 100644
> --- a/arch/score/include/asm/barrier.h
> +++ b/arch/score/include/asm/barrier.h
> @@ -5,6 +5,7 @@
>  #define rmb()		barrier()
>  #define wmb()		barrier()
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  
> diff --git a/arch/sh/include/asm/barrier.h b/arch/sh/include/asm/barrier.h
> index 72c103dae300..f8dce7926432 100644
> --- a/arch/sh/include/asm/barrier.h
> +++ b/arch/sh/include/asm/barrier.h
> @@ -39,11 +39,13 @@
>  
>  #ifdef CONFIG_SMP
>  #define smp_mb()	mb()
> +#define smp_tmb()	mb()
>  #define smp_rmb()	rmb()
>  #define smp_wmb()	wmb()
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #else
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	do { } while(0)
> diff --git a/arch/sparc/include/asm/barrier_32.h b/arch/sparc/include/asm/barrier_32.h
> index c1b76654ee76..1037ce189cee 100644
> --- a/arch/sparc/include/asm/barrier_32.h
> +++ b/arch/sparc/include/asm/barrier_32.h
> @@ -8,6 +8,7 @@
>  #define read_barrier_depends()	do { } while(0)
>  #define set_mb(__var, __value)  do { __var = __value; mb(); } while(0)
>  #define smp_mb()	__asm__ __volatile__("":::"memory")
> +#define smp_tmb()	__asm__ __volatile__("":::"memory")
>  #define smp_rmb()	__asm__ __volatile__("":::"memory")
>  #define smp_wmb()	__asm__ __volatile__("":::"memory")
>  #define smp_read_barrier_depends()	do { } while(0)
> diff --git a/arch/sparc/include/asm/barrier_64.h b/arch/sparc/include/asm/barrier_64.h
> index 95d45986f908..0f3c2fdb86b8 100644
> --- a/arch/sparc/include/asm/barrier_64.h
> +++ b/arch/sparc/include/asm/barrier_64.h
> @@ -34,6 +34,7 @@ do {	__asm__ __volatile__("ba,pt	%%xcc, 1f\n\t" \
>   * memory ordering than required by the specifications.
>   */
>  #define mb()	membar_safe("#StoreLoad")
> +#define tmb()	__asm__ __volatile__("":::"memory")
>  #define rmb()	__asm__ __volatile__("":::"memory")
>  #define wmb()	__asm__ __volatile__("":::"memory")
>  
> @@ -43,10 +44,12 @@ do {	__asm__ __volatile__("ba,pt	%%xcc, 1f\n\t" \
>  
>  #ifdef CONFIG_SMP
>  #define smp_mb()	mb()
> +#define smp_tmb()	tmb()
>  #define smp_rmb()	rmb()
>  #define smp_wmb()	wmb()
>  #else
>  #define smp_mb()	__asm__ __volatile__("":::"memory")
> +#define smp_tmb()	__asm__ __volatile__("":::"memory")
>  #define smp_rmb()	__asm__ __volatile__("":::"memory")
>  #define smp_wmb()	__asm__ __volatile__("":::"memory")
>  #endif
> diff --git a/arch/tile/include/asm/barrier.h b/arch/tile/include/asm/barrier.h
> index a9a73da5865d..cad3c6ae28bf 100644
> --- a/arch/tile/include/asm/barrier.h
> +++ b/arch/tile/include/asm/barrier.h
> @@ -127,11 +127,13 @@ mb_incoherent(void)
>  
>  #ifdef CONFIG_SMP
>  #define smp_mb()	mb()
> +#define smp_tmb()	mb()
>  #define smp_rmb()	rmb()
>  #define smp_wmb()	wmb()
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #else
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	do { } while (0)
> diff --git a/arch/unicore32/include/asm/barrier.h b/arch/unicore32/include/asm/barrier.h
> index a6620e5336b6..8b341fffbda6 100644
> --- a/arch/unicore32/include/asm/barrier.h
> +++ b/arch/unicore32/include/asm/barrier.h
> @@ -18,6 +18,7 @@
>  #define rmb()				barrier()
>  #define wmb()				barrier()
>  #define smp_mb()			barrier()
> +#define smp_tmb()			barrier()
>  #define smp_rmb()			barrier()
>  #define smp_wmb()			barrier()
>  #define read_barrier_depends()		do { } while (0)
> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
> index c6cd358a1eec..480201d83af1 100644
> --- a/arch/x86/include/asm/barrier.h
> +++ b/arch/x86/include/asm/barrier.h
> @@ -86,14 +86,17 @@
>  # define smp_rmb()	barrier()
>  #endif
>  #ifdef CONFIG_X86_OOSTORE
> +# define smp_tmb()	mb()
>  # define smp_wmb() 	wmb()
>  #else
> +# define smp_tmb()	barrier()
>  # define smp_wmb()	barrier()
>  #endif
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #define set_mb(var, value) do { (void)xchg(&var, value); } while (0)
>  #else
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #define smp_read_barrier_depends()	do { } while (0)
> diff --git a/arch/xtensa/include/asm/barrier.h b/arch/xtensa/include/asm/barrier.h
> index ef021677d536..7839db843ea5 100644
> --- a/arch/xtensa/include/asm/barrier.h
> +++ b/arch/xtensa/include/asm/barrier.h
> @@ -20,6 +20,7 @@
>  #error smp_* not defined
>  #else
>  #define smp_mb()	barrier()
> +#define smp_tmb()	barrier()
>  #define smp_rmb()	barrier()
>  #define smp_wmb()	barrier()
>  #endif
> 
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