From: "Ondřej Surý via lttng-dev" <lttng-dev@lists.lttng.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: lttng-dev@lists.lttng.org, paulmck <paulmck@kernel.org>
Subject: Re: [lttng-dev] [PATCH 2/7] Use gcc __atomic builtis for <urcu/uatomic.h> implementation
Date: Mon, 20 Mar 2023 19:28:16 +0100 [thread overview]
Message-ID: <5C9355A4-AE1B-45B5-915E-D41C45A488BE@sury.org> (raw)
In-Reply-To: <6d73ed33-f3fc-2459-a7e9-142b8908ad4f@efficios.com>
> On 20. 3. 2023, at 19:03, Mathieu Desnoyers <mathieu.desnoyers@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.
Ondrej
--
Ondřej Surý (He/Him)
ondrej@sury.org
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next prev parent reply other threads:[~2023-03-20 18:36 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-17 21:37 [lttng-dev] [PATCH 0/7] Replace the custom code with gcc/clang __atomic builtins Ondřej Surý via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 1/7] Require __atomic builtins to build Ondřej Surý via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 2/7] Use gcc __atomic builtis for <urcu/uatomic.h> implementation Ondřej Surý via lttng-dev
2023-03-20 18:03 ` Mathieu Desnoyers via lttng-dev
2023-03-20 18:13 ` Mathieu Desnoyers via lttng-dev
2023-03-20 18:28 ` Ondřej Surý via lttng-dev [this message]
2023-03-20 18:38 ` Mathieu Desnoyers via lttng-dev
2023-03-20 18:41 ` Mathieu Desnoyers via lttng-dev
2023-03-20 19:38 ` Duncan Sands via lttng-dev
2023-03-21 20:26 ` Mathieu Desnoyers via lttng-dev
2023-03-22 8:24 ` Duncan Sands via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 3/7] Use __atomic_thread_fence() for cmm_barrier() Ondřej Surý via lttng-dev
2023-03-20 18:06 ` Mathieu Desnoyers via lttng-dev
2023-03-20 18:14 ` Mathieu Desnoyers via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 4/7] Replace the internal pointer manipulation with __atomic builtins Ondřej Surý via lttng-dev
2023-03-20 18:25 ` Mathieu Desnoyers via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 5/7] Use __atomic builtins to implement CMM_{LOAD, STORE}_SHARED Ondřej Surý via lttng-dev
2023-03-20 18:28 ` Mathieu Desnoyers via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 6/7] Fix: uatomic_or() need retyping to uintptr_t in rculfhash.c Ondřej Surý via lttng-dev
2023-03-20 18:31 ` Mathieu Desnoyers via lttng-dev
2023-03-21 10:15 ` Ondřej Surý via lttng-dev
2023-03-21 14:44 ` Mathieu Desnoyers via lttng-dev
2023-03-21 14:45 ` Mathieu Desnoyers via lttng-dev
2023-03-17 21:37 ` [lttng-dev] [PATCH 7/7] Experiment: Add explicit memory barrier in free_completion() Ondřej Surý via lttng-dev
2023-03-20 18:37 ` Mathieu Desnoyers via lttng-dev
2023-03-21 10:21 ` Ondřej Surý via lttng-dev
2023-03-21 14:46 ` Mathieu Desnoyers via lttng-dev
2023-03-21 14:48 ` Ondřej Surý via lttng-dev
2023-03-21 14:49 ` Mathieu Desnoyers via lttng-dev
2023-03-21 14:59 ` [lttng-dev] TSAN and the tests Ondřej Surý via lttng-dev
2023-03-21 13:30 [lttng-dev] (no subject) Ondřej Surý via lttng-dev
2023-03-21 13:30 ` [lttng-dev] [PATCH 2/7] Use gcc __atomic builtis for <urcu/uatomic.h> implementation Ondřej Surý via lttng-dev
2023-03-21 20:03 ` Mathieu Desnoyers via lttng-dev
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