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Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context
Date 2017-06-06 18:20 +0200
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References <tP8me-aN-23@gated-at.bofh.it> <tP8vT-e0-7@gated-at.bofh.it> <tPknp-7Bq-47@gated-at.bofh.it> <tPmpd-OH-45@gated-at.bofh.it> <tPnXY-1DX-19@gated-at.bofh.it>
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On Tue, Jun 06, 2017 at 04:45:57PM +0200, Christian Borntraeger wrote:
> A the same time, the implicit memory barrier of the atomic_inc should be
> even cheaper. In contrast to x86, a full smp_mb seems to be almost for
> free (looks like <= 1 cycle for a bcr 14,0 and no contention). So I
> _think_ that this should be really fast enough.

So there is a patch out there that changes the x86 smp_mb()
implementation to do "LOCK ADD some_stack_location, 0" which is lots
cheaper than the "MFENCE" instruction and provides similar guarantees.

HPA was running that through some of the architects.. ping?

(Also, I can imagine OoO CPUs collapsing back-to-back ordering stuff,
but what do I know).

> As a side note, I am asking myself, though, why we do need the
> preempt_disable/enable for the cases where we use the opcodes 
> like lao (atomic load and or to a memory location) and friends.

I suspect the real reason is CPU hotplug, because regular preemption
should not matter. It would be the same as getting migrated the moment
_after_ you do the $op.

But preempt_disable() also holds off hotplug and thereby serializes
against hotplug notifiers that want to, for instance, move the value of
the per-cpu variable to a still online CPU. Without this serialization
it would be possible for the $op to happen _after_ the hotplug notifier
runs.

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[PATCH RFC tip/core/rcu 0/2] srcu: All SRCU readers from both  process and irq "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 00:10 +0200
  [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 00:20 +0200
    Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 13:00 +0200
      Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 15:00 +0200
      Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Paolo Bonzini <pbonzini@redhat.com> - 2017-06-06 15:10 +0200
        Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Christian Borntraeger <borntraeger@de.ibm.com> - 2017-06-06 16:50 +0200
          Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Heiko Carstens <heiko.carstens@de.ibm.com> - 2017-06-06 17:30 +0200
            Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Christian Borntraeger <borntraeger@de.ibm.com> - 2017-06-06 17:40 +0200
              Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 18:00 +0200
            Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 18:20 +0200
              Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 19:10 +0200
              Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Heiko Carstens <heiko.carstens@de.ibm.com> - 2017-06-06 19:30 +0200
          Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 18:20 +0200
        Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 18:10 +0200
    Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 13:10 +0200
      Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Paolo Bonzini <pbonzini@redhat.com> - 2017-06-06 14:10 +0200
        Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 15:00 +0200
          Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 18:00 +0200
            Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 18:00 +0200
    Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 19:30 +0200
      Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 20:00 +0200
        Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 20:10 +0200
          Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 20:30 +0200
  [PATCH RFC tip/core/rcu 2/2] srcu: Allow use of Classic SRCU from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 00:20 +0200
  Re: [PATCH RFC tip/core/rcu 0/2] srcu: All SRCU readers from both  process and irq "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 19:10 +0200

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