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

[Peter Zijlstra]

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.

$ ./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