From:   Florian Weimer <fweimer@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     linux-api <linux-api@vger.kernel.org>,
        Jann Horn <jannh@google.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        paulmck <paulmck@kernel.org>
Subject: Re: rseq + membarrier programming model
References: <87tufctk82.fsf@oldenburg.str.redhat.com>
        <697825714.30478.1639423180784.JavaMail.zimbra@efficios.com>
Date:   Mon, 13 Dec 2021 20:29:50 +0100
In-Reply-To: <697825714.30478.1639423180784.JavaMail.zimbra@efficios.com>
        (Mathieu Desnoyers's message of "Mon, 13 Dec 2021 14:19:40 -0500
        (EST)")
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* Mathieu Desnoyers:

>> Could it fall back to
>> MEMBARRIER_CMD_GLOBAL instead?
>
> No. CMD_GLOBAL does not issue the required rseq fence used by the
> algorithm discussed. Also, CMD_GLOBAL has quite a few other shortcomings:
> it takes a while to execute, and is incompatible with nohz_full kernels.

What about using sched_setcpu to move the current thread to the same CPU
(and move it back afterwards)?  Surely that implies the required sort of
rseq barrier that MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ with
MEMBARRIER_CMD_FLAG_CPU performs?

That is possible even without membarrier, so I wonder why registration
of intent is needed for MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ.

> In order to make sure the programming model is the same for expedited
> private/global plain/sync-core/rseq membarrier commands, we require that
> each process perform a registration beforehand.

Hmm.  At least it's not possible to unregister again.

But I think it would be really useful to have some of these barriers
available without registration, possibly in a more expensive form.

Thanks,
Florian