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

From: Paul E. McKenney
Date: Sun Nov 03 2013 - 17:46:48 EST

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On Mon, Nov 04, 2013 at 07:59:23AM +1100, Benjamin Herrenschmidt wrote:
> 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...

Agreed. Also it needs a name that people can live with. We will get
there. ;-)

Thanx, Paul

> 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 <paulmck@xxxxxxxxxxxxxxxxxx>
> > Not-Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > ---
> > 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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