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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 8 Apr 2016 08:58:27 -0700
Message-ID: <CALCETrUi+TXUD3PJqm9BqV+e9ozi7nJDp2u+gpUHBzPxe8Ub9A@mail.gmail.com>
Subject: Re: [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu
 critical sections
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Paul Turner <commonly@gmail.com>,
        Chris Lameter <cl@linux.com>, Andi Kleen <andi@firstfloor.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Dave Watson <davejwatson@fb.com>,
        Linux API <linux-api@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Hunter <ahh@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On Apr 7, 2016 11:41 PM, "Peter Zijlstra" <peterz@infradead.org> wrote:
>
> On Thu, Apr 07, 2016 at 09:43:33AM -0700, Andy Lutomirski wrote:
> > enter the critical section:
> > 1:
> > movq %[cpu], %%r12
> > movq {address of counter for our cpu}, %%r13
> > movq {some fresh value}, (%%r13)
> > cmpq %[cpu], %%r12
> > jne 1b
>
> This is inherently racy; your forgot the detail of 'some fresh value',
> but since you want to avoid collisions you really want an increment.
>
> But load-store archs cannot do that. Or rather, they need to do:
>
>         load    Rn, $event
>         add     Rn, Rn, 1
>         store   $event, Rn
>
> But if they're preempted in the middle, two threads will collide and
> generate the _same_ increment. Comparing CPU numbers will not fix that.

Even on x86 this won't work -- we have no actual guarantee we're on
the right CPU, so we'd have to use an atomic.

I was thinking we'd allocate from a per-thread pool (say 24 bits of
thread ID and the rest being a nonce).  On load-store architectures
this wouldn't be async-signal-safe, though.  Hmm.

--Andy