[lttng-dev] [PATCH 04/11] urcu/arch/generic: Use atomic builtins if configured

[Olivier Dion]

On Wed, 21 Jun 2023, "Paul E. McKenney" <paulmck at kernel.org> wrote:
> On Mon, May 15, 2023 at 04:17:11PM -0400, Olivier Dion wrote:
>>  #ifndef cmm_mb
>>  #define cmm_mb()    __sync_synchronize()
>
> Just out of curiosity, why not also implement cmm_mb() in terms of
> __atomic_thread_fence(__ATOMIC_SEQ_CST)?  (Or is that a later patch?)

IIRC, Mathieu and I agree that the definition of a thread fence -- acts
as a synchronization fence between threads -- is too weak for what we
want here.  For example, with I/O devices.

Although __sync_synchronize() is probably an alias for a SEQ_CST thread
fence, its definition -- issues a full memory barrier -- is stronger.

We do not want to rely on this assumption (alias) and prefer to rely on
the documented definition instead.

-- 
Olivier Dion
EfficiOS Inc.
https://www.efficios.com