Re: [Qemu-devel] [RFC PATCH 3/4] include/qemu/atomic.h: default to __atomic functions

[Paolo Bonzini <pbonzini@redhat.com>]

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