Re: [lttng-dev] [PATCH 2/7] Use gcc __atomic builtis for <urcu/uatomic.h> implementation

[Mathieu Desnoyers via lttng-dev <lttng-dev@lists.lttng.org>]

On 2023-03-20 14:03, Mathieu Desnoyers via lttng-dev wrote:
> On 2023-03-17 17:37, Ondřej Surý via lttng-dev wrote:
>> Replace the custom assembly code in include/urcu/uatomic/ with __atomic
>> builtins provided by C11-compatible compiler.
>>
> [...]
>> +#define UATOMIC_HAS_ATOMIC_BYTE
>> +#define UATOMIC_HAS_ATOMIC_SHORT
>> +
>> +#define uatomic_set(addr, v) __atomic_store_n(addr, v, __ATOMIC_RELEASE)
>> +
>> +#define uatomic_read(addr) __atomic_load_n((addr), __ATOMIC_CONSUME)
>> +
>> +#define uatomic_xchg(addr, v) __atomic_exchange_n((addr), (v), 
>> __ATOMIC_ACQ_REL)
>> +
>> +#define uatomic_cmpxchg(addr, old, new) \
>> +    ({                                    \
>> +        __typeof__(*(addr)) __old = old;                \
>> +        __atomic_compare_exchange_n(addr, &__old, new, 0,        \
>> +                        __ATOMIC_ACQ_REL, __ATOMIC_CONSUME);\
> 

Actually, I suspect we'd want to change __ATOMIC_ACQ_REL to 
__ATOMIC_SEQ_CST everywhere, because we want total order.

Thanks,

Mathieu

> In doc/uatomic-api.md, we document:
> 
> "```c
> type uatomic_cmpxchg(type *addr, type old, type new);
> ```
> 
> An atomic read-modify-write operation that performs this
> sequence of operations atomically: check if `addr` contains `old`.
> If true, then replace the content of `addr` by `new`. Return the
> value previously contained by `addr`. This function implies a full
> memory barrier before and after the atomic operation."
> 
> This would map to a "__ATOMIC_ACQ_REL" semantic on cmpxchg failure
> rather than __ATOMIC_CONSUME".
> 
>> +        __old;                                \
>> +    })
>> +
>> +#define uatomic_add_return(addr, v) \
>> +    __atomic_add_fetch((addr), (v), __ATOMIC_ACQ_REL)
>> +
>> +#define uatomic_add(addr, v) \
>> +    (void)__atomic_add_fetch((addr), (v), __ATOMIC_RELAXED)
>> +
>> +#define uatomic_sub_return(addr, v) \
>> +    __atomic_sub_fetch((addr), (v), __ATOMIC_ACQ_REL)
>> +
>> +#define uatomic_sub(addr, v) \
>> +    (void)__atomic_sub_fetch((addr), (v), __ATOMIC_RELAXED)
>> +
>> +#define uatomic_and(addr, mask) \
>> +    (void)__atomic_and_fetch((addr), (mask), __ATOMIC_RELAXED)
>> +
>> +#define uatomic_or(addr, mask)                        \
>> +    (void)__atomic_or_fetch((addr), (mask), __ATOMIC_RELAXED)
>> +
>> +#define uatomic_inc(addr) (void)__atomic_add_fetch((addr), 1, 
>> __ATOMIC_RELAXED)
>> +#define uatomic_dec(addr) (void)__atomic_sub_fetch((addr), 1, 
>> __ATOMIC_RELAXED)
>> +
>> +#define cmm_smp_mb__before_uatomic_and()    
>> __atomic_thread_fence(__ATOMIC_ACQ_REL)
>> +#define cmm_smp_mb__after_uatomic_and()        
>> __atomic_thread_fence(__ATOMIC_ACQ_REL)
>> +#define cmm_smp_mb__before_uatomic_or()        
>> __atomic_thread_fence(__ATOMIC_ACQ_REL)
>> +#define cmm_smp_mb__after_uatomic_or()        
>> __atomic_thread_fence(__ATOMIC_ACQ_REL)
>> +#define cmm_smp_mb__before_uatomic_add()    
>> __atomic_thread_fence(__ATOMIC_ACQ_REL)
>> +#define cmm_smp_mb__after_uatomic_add()        
>> __atomic_thread_fence(__ATOMIC_ACQ_REL)
>> +#define cmm_smp_mb__before_uatomic_sub()    
>> cmm_smp_mb__before_uatomic_add()
>> +#define cmm_smp_mb__after_uatomic_sub()        
>> cmm_smp_mb__after_uatomic_add()
>> +#define cmm_smp_mb__before_uatomic_inc()    
>> cmm_smp_mb__before_uatomic_add()
>> +#define cmm_smp_mb__after_uatomic_inc()        
>> cmm_smp_mb__after_uatomic_add()
>> +#define cmm_smp_mb__before_uatomic_dec()    
>> cmm_smp_mb__before_uatomic_add()
>> +#define cmm_smp_mb__after_uatomic_dec()        
>> cmm_smp_mb__after_uatomic_add()
>> +
>> +#define cmm_smp_mb()                cmm_mb()
> 
> While OK for the general case, I would recommend that we immediately 
> implement something more efficient on x86 32/64 which takes into account 
> that __ATOMIC_ACQ_REL atomic operations are implemented with LOCK 
> prefixed atomic ops, which imply the barrier already, leaving the 
> before/after_uatomic_*() as no-ops.
> 
> Thanks,
> 
> Mathieu
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

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