Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()

[Paul E. McKenney]

On Tue, Apr 23, 2019 at 02:17:15PM +0200, Peter Zijlstra wrote:
> On Sat, Apr 20, 2019 at 01:54:40AM -0700, Paul E. McKenney wrote:
> > 3.	Make non-value-returning atomics provide full ordering.
> > 	This would of course need some benchmarking, but would be a
> > 	simple change to make and would eliminate a large class of
> > 	potential bugs.  My guess is that the loss in performance
> > 	would be non-negligible, but who knows?
> 
> Well, only for the architectures that have
> smp_mb__{before,after}_atomic() as barrier(), which are: ia64, mips,
> s390, sparc, x86 and xtense.

The weakly ordered architectures would need to add the equivalent of
smp_mb() before and after, right?  This might result in a more noticeable
loss of performance.

							Thanx, Paul

> $ ./compare.sh defconfig-build defconfig-build1 vmlinux
> do_profile_hits                                           275        278   +3,+0
> freezer_apply_state                                        86         98   +12,+0
> perf_event_alloc                                         2232       2261   +29,+0
> _free_event                                               631        660   +29,+0
> shmem_add_to_page_cache                                   712        722   +10,+0
> _enable_swap_info                                         333        337   +4,+0
> do_mmu_notifier_register                                  303        311   +8,+0
> __nfs_commit_inode                                        356        359   +3,+0
> tcp_try_coalesce                                          246        250   +4,+0
> i915_gem_free_object                                       90         97   +7,+0
> mce_intel_hcpu_update                                      39         47   +8,+0
> __ia32_sys_swapoff                                       1177       1181   +4,+0
> pci_enable_ats                                            124        131   +7,+0
> __x64_sys_swapoff                                        1178       1182   +4,+0
> i915_gem_madvise_ioctl                                    447        443   -4,+0
> calc_global_load_tick                                      75         82   +7,+0
> i915_gem_object_set_tiling                                712        708   -4,+0
> 					     total   11374236   11374367   +131,+0
> Which doesn't look too bad.
> 
> ---
> diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
> index ea3d95275b43..115127c7ad28 100644
> --- a/arch/x86/include/asm/atomic.h
> +++ b/arch/x86/include/asm/atomic.h
> @@ -54,7 +54,7 @@ static __always_inline void arch_atomic_add(int i, atomic_t *v)
>  {
>  	asm volatile(LOCK_PREFIX "addl %1,%0"
>  		     : "+m" (v->counter)
> -		     : "ir" (i));
> +		     : "ir" (i) : "memory");
>  }
>  
>  /**
> @@ -68,7 +68,7 @@ static __always_inline void arch_atomic_sub(int i, atomic_t *v)
>  {
>  	asm volatile(LOCK_PREFIX "subl %1,%0"
>  		     : "+m" (v->counter)
> -		     : "ir" (i));
> +		     : "ir" (i) : "memory");
>  }
>  
>  /**
> @@ -95,7 +95,7 @@ static __always_inline bool arch_atomic_sub_and_test(int i, atomic_t *v)
>  static __always_inline void arch_atomic_inc(atomic_t *v)
>  {
>  	asm volatile(LOCK_PREFIX "incl %0"
> -		     : "+m" (v->counter));
> +		     : "+m" (v->counter) :: "memory");
>  }
>  #define arch_atomic_inc arch_atomic_inc
>  
> @@ -108,7 +108,7 @@ static __always_inline void arch_atomic_inc(atomic_t *v)
>  static __always_inline void arch_atomic_dec(atomic_t *v)
>  {
>  	asm volatile(LOCK_PREFIX "decl %0"
> -		     : "+m" (v->counter));
> +		     : "+m" (v->counter) :: "memory");
>  }
>  #define arch_atomic_dec arch_atomic_dec
>  
> diff --git a/arch/x86/include/asm/atomic64_64.h b/arch/x86/include/asm/atomic64_64.h
> index dadc20adba21..5e86c0d68ac1 100644
> --- a/arch/x86/include/asm/atomic64_64.h
> +++ b/arch/x86/include/asm/atomic64_64.h
> @@ -45,7 +45,7 @@ static __always_inline void arch_atomic64_add(long i, atomic64_t *v)
>  {
>  	asm volatile(LOCK_PREFIX "addq %1,%0"
>  		     : "=m" (v->counter)
> -		     : "er" (i), "m" (v->counter));
> +		     : "er" (i), "m" (v->counter) : "memory");
>  }
>  
>  /**
> @@ -59,7 +59,7 @@ static inline void arch_atomic64_sub(long i, atomic64_t *v)
>  {
>  	asm volatile(LOCK_PREFIX "subq %1,%0"
>  		     : "=m" (v->counter)
> -		     : "er" (i), "m" (v->counter));
> +		     : "er" (i), "m" (v->counter) : "memory");
>  }
>  
>  /**
> @@ -87,7 +87,7 @@ static __always_inline void arch_atomic64_inc(atomic64_t *v)
>  {
>  	asm volatile(LOCK_PREFIX "incq %0"
>  		     : "=m" (v->counter)
> -		     : "m" (v->counter));
> +		     : "m" (v->counter) : "memory");
>  }
>  #define arch_atomic64_inc arch_atomic64_inc
>  
> @@ -101,7 +101,7 @@ static __always_inline void arch_atomic64_dec(atomic64_t *v)
>  {
>  	asm volatile(LOCK_PREFIX "decq %0"
>  		     : "=m" (v->counter)
> -		     : "m" (v->counter));
> +		     : "m" (v->counter) : "memory");
>  }
>  #define arch_atomic64_dec arch_atomic64_dec
>  
>