From: Paolo Bonzini <pbonzini@redhat.com>
To: "Alex Bennée" <alex.bennee@linaro.org>, qemu-devel@nongnu.org
Cc: mttcg@greensocs.com, peter.maydell@linaro.org,
mark.burton@greensocs.com, a.rigo@virtualopensystems.com,
fred.konrad@greensocs.com
Subject: Re: [Qemu-devel] [RFC PATCH 3/4] include/qemu/atomic.h: default to __atomic functions
Date: Mon, 25 Jan 2016 19:04:48 +0100 [thread overview]
Message-ID: <56A663C0.80806@redhat.com> (raw)
In-Reply-To: <1453740558-16303-4-git-send-email-alex.bennee@linaro.org>
On 25/01/2016 17:49, Alex Bennée wrote:
> The __atomic primitives have been available since GCC 4.7 and provide
> a richer interface for describing memory ordering requirements. As a
> bonus by using the primitives instead of hand-rolled functions we can
> use tools such as the AddressSanitizer which need the use of well
> defined APIs for its analysis.
>
> If we have __ATOMIC defines we exclusively use the __atomic primitives
> for all our atomic access. Otherwise we fall back to the mixture of
> __sync and hand-rolled barrier cases.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
> include/qemu/atomic.h | 126 ++++++++++++++++++++++++++++++++------------------
> 1 file changed, 82 insertions(+), 44 deletions(-)
>
> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
> index bd2c075..414c81a 100644
> --- a/include/qemu/atomic.h
> +++ b/include/qemu/atomic.h
> @@ -15,12 +15,90 @@
>
> #include "qemu/compiler.h"
>
> -/* For C11 atomic ops */
>
> /* Compiler barrier */
> #define barrier() ({ asm volatile("" ::: "memory"); (void)0; })
>
> -#ifndef __ATOMIC_RELAXED
> +#ifdef __ATOMIC_RELAXED
> +/* For C11 atomic ops */
> +
> +/* Manual memory barriers
> + *
> + *__atomic_thread_fence does not include a compiler barrier; instead,
> + * the barrier is part of __atomic_load/__atomic_store's "volatile-like"
> + * semantics. If smp_wmb() is a no-op, absence of the barrier means that
> + * the compiler is free to reorder stores on each side of the barrier.
> + * Add one here, and similarly in smp_rmb() and smp_read_barrier_depends().
> + */
> +
> +#define smp_mb() ({ barrier(); __atomic_thread_fence(__ATOMIC_CONSUME); barrier(); })
This should be seq_cst.
> +#define smp_wmb() ({ barrier(); __atomic_thread_fence(__ATOMIC_RELEASE); barrier(); })
> +#define smp_rmb() ({ barrier(); __atomic_thread_fence(__ATOMIC_ACQUIRE); barrier(); })
> +
> +#define smp_read_barrier_depends() ({ barrier(); __atomic_thread_fence(__ATOMIC_CONSUME); barrier(); })
> +
> +/* Weak atomic operations prevent the compiler moving other
> + * loads/stores past the atomic operation load/store.
> + */
> +#define atomic_read(ptr) \
> + ({ \
> + typeof(*ptr) _val; \
> + __atomic_load(ptr, &_val, __ATOMIC_CONSUME); \
This should be relaxed.
> + _val; \
> + })
> +
> +#define atomic_rcu_read(ptr) atomic_read(ptr)
This should be consume.
> +
> +#define atomic_set(ptr, i) do { \
> + typeof(*ptr) _val = (i); \
> + __atomic_store(ptr, &_val, __ATOMIC_RELEASE); \
This should be relaxed.
> +} while(0)
> +
> +#define atomic_rcu_set(ptr, i) atomic_set(ptr, i)
This should be release.
> +/* Sequentially consistent atomic access */
> +
> +#define atomic_xchg(ptr, i) ({ \
> + typeof(*ptr) _new = (i), _old; \
> + __atomic_exchange(ptr, &_new, &_old, __ATOMIC_SEQ_CST); \
> + _old; \
> +})
> +
> +/* Returns the eventual value, failed or not */
> +#define atomic_cmpxchg(ptr, old, new) \
> + ({ \
> + typeof(*ptr) _old = (old), _new = (new); \
> + __atomic_compare_exchange(ptr, &_old, &_new, false, \
> + __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); \
> + *ptr; /* can this race if cmpxchg not used elsewhere? */ \
If I read the manual correctly, you can return _old here (that's why it
is a pointer).
> + })
> +
> +#define atomic_mb_set(ptr, i) ((void)atomic_xchg(ptr, i))
> +#define atomic_mb_read(ptr) \
> + ({ \
> + typeof(*ptr) _val; \
> + __atomic_load(ptr, &_val, __ATOMIC_ACQUIRE); \
> + _val; \
> + })
Please leave these defined in terms of relaxed accesses and memory barriers.
Paolo
next prev parent reply other threads:[~2016-01-25 18:04 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-25 16:49 [Qemu-devel] [RFC PATCH 0/4] ThreadSanitizer support Alex Bennée
2016-01-25 16:49 ` [Qemu-devel] [RFC PATCH 1/4] configure: move EXTRA_CFLAGS append to the end Alex Bennée
2016-01-25 17:04 ` Peter Maydell
2016-01-25 17:37 ` Peter Maydell
2016-01-25 16:49 ` [Qemu-devel] [RFC PATCH 2/4] configure: introduce --extra-libs Alex Bennée
2016-01-25 17:08 ` Peter Maydell
2016-01-25 17:25 ` Alex Bennée
2016-01-25 17:36 ` Peter Maydell
2016-01-25 17:58 ` Paolo Bonzini
2016-01-25 18:15 ` Alex Bennée
2016-01-25 22:10 ` Paolo Bonzini
2016-01-26 8:43 ` Alex Bennée
2016-01-25 16:49 ` [Qemu-devel] [RFC PATCH 3/4] include/qemu/atomic.h: default to __atomic functions Alex Bennée
2016-01-25 18:04 ` Paolo Bonzini [this message]
2016-01-25 18:23 ` Alex Bennée
2016-01-25 22:02 ` Paolo Bonzini
2016-01-25 16:49 ` [Qemu-devel] [RFC PATCH 4/4] tsan: various fixes for make check Alex Bennée
2016-01-25 18:09 ` Paolo Bonzini
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