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

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Mon Mar 20 14:38:54 EDT 2023

On 2023-03-20 14:28, Ondřej Surý wrote:
>> On 20. 3. 2023, at 19:03, Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote:
>> 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".
> From: https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
>> If desired is written into *ptr then true is returned and memory is affected according to the memory order specified by success_memorder. There are no restrictions on what memory order can be used here.
>> Otherwise, false is returned and memory is affected according to failure_memorder. This memory order cannot be __ATOMIC_RELEASE nor __ATOMIC_ACQ_REL. It also cannot be a stronger order than that specified by success_memorder.
> I think it makes sense that the failure_memorder has the same memorder as uatomic_read(), but it definitelly cannot be __ATOMIC_ACQ_REL - it's same as with __atomic_load_n, only following are permitted:
>> The valid memory order variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, __ATOMIC_ACQUIRE, and __ATOMIC_CONSUME.

Based on my other reply, we want "SEQ_CST" rather than ACQ_REL everywhere.



> Ondrej
> --
> Ondřej Surý (He/Him)
> ondrej at sury.org

Mathieu Desnoyers
EfficiOS Inc.

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