On 2023-03-20 14:38, Mathieu Desnoyers via lttng-dev wrote:
On 2023-03-20 14:28, Ondřej Surý wrote:
On 20. 3. 2023, at 19:03, Mathieu Desnoyers
<mathieu.desnoy...@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.
And it _would_ make sense to use the same memorder on cmpxchg failure as
uatomic_read if we were exposing a new API, but we are modifying an
already exposed documented API, so I would stick to SEQ_CST for both
cmpxchg success/failure.
If we want to expose a new cmpxchg_relaxed_failure with a relaxed
memorder on failure that would be fine, but we cannot change the
semantic that is already documented.
Thanks,
Mathieu
Thanks,
Mathieu
Ondrej
--
Ondřej Surý (He/Him)
ond...@sury.org
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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