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McKenney" Cc: Nicholas Piggin , LKMM Maintainers -- Akira Yokosawa , Andrea Parri , Boqun Feng , David Howells , Daniel Lustig , Jade Alglave , Kernel development list , Luc Maranget , Alan Stern , Will Deacon Subject: Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic() Message-ID: <20190423121715.GQ4038@hirez.programming.kicks-ass.net> References: <20190419180017.GP4038@hirez.programming.kicks-ass.net> <20190419182620.GF14111@linux.ibm.com> <1555719429.t9n8gkf70y.astroid@bobo.none> <20190420085440.GK14111@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190420085440.GK14111@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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