x86: optimise barriers
According to latest memory ordering specification documents from Intel and AMD, both manufacturers are committed to in-order loads from cacheable memory for the x86 architecture. Hence, smp_rmb() may be a simple barrier. Also according to those documents, and according to existing practice in Linux (eg. spin_unlock doesn't enforce ordering), stores to cacheable memory are visible in program order too. Special string stores are safe -- their constituent stores may be out of order, but they must complete in order WRT surrounding stores. Nontemporal stores to WB memory can go out of order, and so they should be fenced explicitly to make them appear in-order WRT other stores. Hence, smp_wmb() may be a simple barrier. http://developer.intel.com/products/processor/manuals/318147.pdf http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/24593.pdf In userspace microbenchmarks on a core2 system, fence instructions range anywhere from around 15 cycles to 50, which may not be totally insignificant in performance critical paths (code size will go down too). However the primary motivation for this is to have the canonical barrier implementation for x86 architecture. smp_rmb on buggy pentium pros remains a locked op, which is apparently required. Signed-off-by: Nick Piggin <npiggin@suse.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -274,7 +274,11 @@ static inline unsigned long get_limit(unsigned long segment)
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#ifdef CONFIG_SMP
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#ifdef CONFIG_SMP
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#define smp_mb() mb()
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#define smp_mb() mb()
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#define smp_rmb() rmb()
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#ifdef CONFIG_X86_PPRO_FENCE
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# define smp_rmb() rmb()
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#else
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# define smp_rmb() barrier()
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#endif
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#ifdef CONFIG_X86_OOSTORE
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#ifdef CONFIG_X86_OOSTORE
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# define smp_wmb() wmb()
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# define smp_wmb() wmb()
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#else
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#else
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@ -141,8 +141,8 @@ static inline void write_cr8(unsigned long val)
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#ifdef CONFIG_SMP
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#ifdef CONFIG_SMP
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#define smp_mb() mb()
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#define smp_mb() mb()
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#define smp_rmb() rmb()
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#define smp_rmb() barrier()
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#define smp_wmb() wmb()
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#define smp_wmb() barrier()
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#define smp_read_barrier_depends() do {} while(0)
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#define smp_read_barrier_depends() do {} while(0)
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#else
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#else
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#define smp_mb() barrier()
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#define smp_mb() barrier()
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