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Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics |
Date: |
Sun, 22 May 2016 08:58:51 +0100 |
User-agent: |
mu4e 0.9.17; emacs 25.0.93.7 |
Emilio G. Cota <address@hidden> writes:
> Commit a0aa44b4 ("include/qemu/atomic.h: default to __atomic functions")
> set all atomics to default (on recent GCC versions) to __atomic primitives.
>
> In the process, the atomic_rcu_read/set were converted to implement
> consume/release semantics, respectively. This is inefficient; for
> correctness and maximum performance we only need an smp_barrier_depends
> for reads, and an smp_wmb for writes. Fix it by using the original
> definition of these two primitives for all compilers.
>
> Note that in case these semantics were necessary to avoid false
> positives under Thread Sanitizer, we could have them defined as such
> under #ifdef __SANITIZE_THREAD__. I tried running QEMU under tsan
> to explore this, but unfortunately I couldn't get it to work (tsan dies
> with an "unexpected memory mapping"). I suspect though that the
> atomic_read/set embedded in atomic_rcu_read/set is enough for tsan,
> though.
For tsan runs you need to re-build with:
./configure --cc=gcc --extra-cflags="-pie -fPIE -fsanitize=thread"
--with-coroutine=gthread
Specifically the coroutine ucontext messing really confuses TSAN.
>
> Signed-off-by: Emilio G. Cota <address@hidden>
> ---
> include/qemu/atomic.h | 99
> ++++++++++++++++++++++-----------------------------
> 1 file changed, 43 insertions(+), 56 deletions(-)
>
> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
> index 5bc4d6c..4d62425 100644
> --- a/include/qemu/atomic.h
> +++ b/include/qemu/atomic.h
> @@ -56,22 +56,6 @@
> __atomic_store(ptr, &_val, __ATOMIC_RELAXED); \
> } while(0)
>
> -/* Atomic RCU operations imply weak memory barriers */
> -
> -#define atomic_rcu_read(ptr) \
> - ({ \
> - QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
> - typeof(*ptr) _val; \
> - __atomic_load(ptr, &_val, __ATOMIC_CONSUME); \
> - _val; \
> - })
> -
> -#define atomic_rcu_set(ptr, i) do { \
> - QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
> - typeof(*ptr) _val = (i); \
> - __atomic_store(ptr, &_val, __ATOMIC_RELEASE); \
> -} while(0)
> -
> /* atomic_mb_read/set semantics map Java volatile variables. They are
> * less expensive on some platforms (notably POWER & ARMv7) than fully
> * sequentially consistent operations.
> @@ -242,46 +226,6 @@
> #define atomic_read(ptr) (*(__typeof__(*ptr) volatile*) (ptr))
> #define atomic_set(ptr, i) ((*(__typeof__(*ptr) volatile*) (ptr)) = (i))
>
> -/**
> - * atomic_rcu_read - reads a RCU-protected pointer to a local variable
> - * into a RCU read-side critical section. The pointer can later be safely
> - * dereferenced within the critical section.
> - *
> - * This ensures that the pointer copy is invariant thorough the whole
> critical
> - * section.
> - *
> - * Inserts memory barriers on architectures that require them (currently only
> - * Alpha) and documents which pointers are protected by RCU.
> - *
> - * atomic_rcu_read also includes a compiler barrier to ensure that
> - * value-speculative optimizations (e.g. VSS: Value Speculation
> - * Scheduling) does not perform the data read before the pointer read
> - * by speculating the value of the pointer.
> - *
> - * Should match atomic_rcu_set(), atomic_xchg(), atomic_cmpxchg().
> - */
> -#define atomic_rcu_read(ptr) ({ \
> - typeof(*ptr) _val = atomic_read(ptr); \
> - smp_read_barrier_depends(); \
> - _val; \
> -})
> -
> -/**
> - * atomic_rcu_set - assigns (publicizes) a pointer to a new data structure
> - * meant to be read by RCU read-side critical sections.
> - *
> - * Documents which pointers will be dereferenced by RCU read-side critical
> - * sections and adds the required memory barriers on architectures requiring
> - * them. It also makes sure the compiler does not reorder code initializing
> the
> - * data structure before its publication.
> - *
> - * Should match atomic_rcu_read().
> - */
> -#define atomic_rcu_set(ptr, i) do { \
> - smp_wmb(); \
> - atomic_set(ptr, i); \
> -} while (0)
> -
> /* These have the same semantics as Java volatile variables.
> * See http://gee.cs.oswego.edu/dl/jmm/cookbook.html:
> * "1. Issue a StoreStore barrier (wmb) before each volatile store."
> @@ -345,4 +289,47 @@
> #define atomic_or(ptr, n) ((void) __sync_fetch_and_or(ptr, n))
>
> #endif /* __ATOMIC_RELAXED */
> +
> +/**
> + * atomic_rcu_read - reads a RCU-protected pointer to a local variable
> + * into a RCU read-side critical section. The pointer can later be safely
> + * dereferenced within the critical section.
> + *
> + * This ensures that the pointer copy is invariant thorough the whole
> critical
> + * section.
> + *
> + * Inserts memory barriers on architectures that require them (currently only
> + * Alpha) and documents which pointers are protected by RCU.
> + *
> + * atomic_rcu_read also includes a compiler barrier to ensure that
> + * value-speculative optimizations (e.g. VSS: Value Speculation
> + * Scheduling) does not perform the data read before the pointer read
> + * by speculating the value of the pointer.
> + *
> + * Should match atomic_rcu_set(), atomic_xchg(), atomic_cmpxchg().
> + */
> +#define atomic_rcu_read(ptr) ({ \
> + QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
> + typeof(*ptr) _val = atomic_read(ptr); \
> + smp_read_barrier_depends(); \
> + _val; \
> +})
> +
> +/**
> + * atomic_rcu_set - assigns (publicizes) a pointer to a new data structure
> + * meant to be read by RCU read-side critical sections.
> + *
> + * Documents which pointers will be dereferenced by RCU read-side critical
> + * sections and adds the required memory barriers on architectures requiring
> + * them. It also makes sure the compiler does not reorder code initializing
> the
> + * data structure before its publication.
> + *
> + * Should match atomic_rcu_read().
> + */
> +#define atomic_rcu_set(ptr, i) do { \
> + QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
> + smp_wmb(); \
> + atomic_set(ptr, i); \
> +} while (0)
> +
> #endif /* __QEMU_ATOMIC_H */
--
Alex Bennée
- [Qemu-devel] [PATCH 0/2] atomics: fix small RCU perf. regression + update documentation, Emilio G. Cota, 2016/05/21
- [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Emilio G. Cota, 2016/05/21
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics,
Alex Bennée <=
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Paolo Bonzini, 2016/05/23
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Richard Henderson, 2016/05/23
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Emilio G. Cota, 2016/05/23
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Paolo Bonzini, 2016/05/24
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Emilio G. Cota, 2016/05/24
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Sergey Fedorov, 2016/05/24
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Alex Bennée, 2016/05/25
- Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics, Sergey Fedorov, 2016/05/25