Message-ID: <1440811986.1831.9.camel@stgolabs.net>
Subject: Re: futex atomic vs ordering constraints
From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, mtk.manpages@gmail.com,
        dvhart@infradead.org, Vineet.Gupta1@synopsys.com, ralf@linux-mips.org,
        ddaney@caviumnetworks.com, Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org
Date: Fri, 28 Aug 2015 18:33:06 -0700
In-Reply-To: <20150826181659.GW16853@twins.programming.kicks-ass.net>
References: <20150826181659.GW16853@twins.programming.kicks-ass.net>
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On Wed, 2015-08-26 at 20:16 +0200, Peter Zijlstra wrote:
> Of course, if anything else prior to futex_atomic_op_inuser() implies an
> (RCsc) RELEASE or stronger the primitive can do without providing
> anything itself.
> 
> This turns out to be the case, a successful get_futex_key() implies a
> full memory barrier; recent: 1d0dcb3ad9d3 ("futex: Implement lockless
> wakeups").

Hmm while it is certainly true that get_futex_key() implies a full
barrier, I don't see why you're referring to the recent wake_q stuff;
where the futex "wakeup" is done much after futex_atomic_op_inuser. Yes,
that too implies a barrier, but not wrt get_futex_key() -- which
fundamentally relies on get_futex_key_refs().

> 
> And since get_futex_key() is fundamental to doing _anything_ with a
> futex, I think its semi-sane to rely on this.

Right, and it wouldn't be the first thing that relies on get_futex_key()
implying a full barrier.

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