atomics: do not emit consume barrier for atomic_rcu_read
Currently we emit a consume-load in atomic_rcu_read. Because of limitations in current compilers, this is overkill for non-Alpha hosts and it is only useful to make Thread Sanitizer work. This patch leaves the consume-load in atomic_rcu_read when compiling with Thread Sanitizer enabled, and resorts to a relaxed load + smp_read_barrier_depends otherwise. On an RMO host architecture, such as aarch64, the performance improvement of this change is easily measurable. For instance, qht-bench performs an atomic_rcu_read on every lookup. Performance before and after applying this patch: $ tests/qht-bench -d 5 -n 1 Before: 9.78 MT/s After: 10.96 MT/s Signed-off-by: Emilio G. Cota <cota@braap.org> Message-Id: <1464120374-8950-4-git-send-email-cota@braap.org> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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@ -67,13 +67,23 @@
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__atomic_store(ptr, &_val, __ATOMIC_RELAXED); \
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__atomic_store(ptr, &_val, __ATOMIC_RELAXED); \
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} while(0)
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} while(0)
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/* Atomic RCU operations imply weak memory barriers */
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/* See above: most compilers currently treat consume and acquire the
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* same, but this slows down atomic_rcu_read unnecessarily.
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*/
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#ifdef __SANITIZE_THREAD__
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#define atomic_rcu_read__nocheck(ptr, valptr) \
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__atomic_load(ptr, valptr, __ATOMIC_CONSUME);
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#else
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#define atomic_rcu_read__nocheck(ptr, valptr) \
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__atomic_load(ptr, valptr, __ATOMIC_RELAXED); \
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smp_read_barrier_depends();
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#endif
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#define atomic_rcu_read(ptr) \
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#define atomic_rcu_read(ptr) \
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({ \
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({ \
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QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
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QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
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typeof(*ptr) _val; \
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typeof(*ptr) _val; \
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__atomic_load(ptr, &_val, __ATOMIC_CONSUME); \
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atomic_rcu_read__nocheck(ptr, &_val); \
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_val; \
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_val; \
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})
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})
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