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Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option
Date 2017-07-27 17:30 +0200
Message-ID <u7STF-7Md-27@gated-at.bofh.it> (permalink)
References (5 earlier) <u7peW-5RS-17@gated-at.bofh.it> <u7wJs-2ak-21@gated-at.bofh.it> <u7MuR-3NX-9@gated-at.bofh.it> <u7QIb-6w8-23@gated-at.bofh.it> <u7Ruy-6NU-17@gated-at.bofh.it>
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Hi Nick,

See below,

On Thu, Jul 27, 2017 at 03:56:10PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 27, 2017 at 06:08:16AM -0700, Paul E. McKenney wrote:
> 
> > > So I think we need either switch_mm() or switch_to() to imply a full
> > > barrier for this to work, otherwise we get:
> > > 
> > >   CPU0				CPU1
> > > 
> > > 
> > >   lock rq->lock
> > >   mb
> > > 
> > >   rq->curr = A
> > > 
> > >   unlock rq->lock
> > > 
> > >   lock rq->lock
> > >   mb
> > > 
> > > 				sys_membarrier()
> > > 
> > > 				mb
> > > 
> > > 				for_each_online_cpu()
> > > 				  p = A
> > > 				  // no match no IPI
> > > 
> > > 				mb
> > >   rq->curr = B
> > > 
> > >   unlock rq->lock
> > > 
> > > 
> > > And that's bad, because now CPU0 doesn't have an MB happening _after_
> > > sys_membarrier() if B matches.
> > 
> > Yes, this looks somewhat similar to the scenario that Mathieu pointed out
> > back in 2010: https://marc.info/?l=linux-kernel&m=126349766324224&w=2
> 
> Yes. Minus the mm_cpumask() worries.
> 
> > > So without audit, I only know of PPC and Alpha not having a barrier in
> > > either switch_*().
> > > 
> > > x86 obviously has barriers all over the place, arm has a super duper
> > > heavy barrier in switch_to().
> > 
> > Agreed, if we are going to rely on ->mm, we need ordering on assignment
> > to it.
> 
> Right, Boqun provided this reordering to show the problem:
> 
>   CPU0                                CPU1
>  
>  
>   <in process X>
>   lock rq->lock
>   mb
>  
>   rq->curr = A
>  
>   unlock rq->lock
>  
>   <switch to process A>
>  
>   lock rq->lock
>   mb
>   read Y(reordered)<---+
>                        |        store to Y
>                        |
>                        |        sys_membarrier()
>                        |
>                        |        mb
>                        |
>                        |        for_each_online_cpu()
>                        |          p = A
>                        |          // no match no IPI
>                        |
>                        |        mb
>                        |
>                        |        store to X
>   rq->curr = B         |
>                        |
>   unlock rq->lock      |
>   <switch to B>        |
>   read X               |
>                        |
>   read Y --------------+

In order to make this work we need either switch_to() or switch_mm() to
provide smp_mb(). Now you're recently taken that out on PPC and I'm
thinking you're not keen to have to put it back in.

Mathieu was wondering if placing it in switch_mm() would be less onerous
on performance, thinking that address space changes are more expensive
in any case, seeing how they have a tail of cache and translation
misses. I'm thinking you're not happy either way :-)

Opinions?

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Thread

[PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 00:00 +0200
  Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Boqun Feng <boqun.feng@gmail.com> - 2017-07-25 06:30 +0200
    Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 18:30 +0200
  Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-25 15:20 +0200
    Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 18:50 +0200
  Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-25 18:40 +0200
    Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 18:50 +0200
      Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-25 19:10 +0200
        Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 19:20 +0200
          Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-25 21:00 +0200
            Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 21:40 +0200
              Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-25 22:30 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-25 23:20 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-26 00:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-26 00:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-26 00:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-26 02:10 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-26 09:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-26 17:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-26 20:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-26 20:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-26 22:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-26 23:20 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 03:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-27 14:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 16:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 12:30 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 17:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 11:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 12:20 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Will Deacon <will.deacon@arm.com> - 2017-07-27 12:30 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 15:20 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-26 02:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-26 09:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-26 17:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 10:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 15:10 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 15:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 16:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 16:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 16:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 16:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 17:30 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Boqun Feng <boqun.feng@gmail.com> - 2017-07-27 16:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 16:20 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Boqun Feng <boqun.feng@gmail.com> - 2017-07-27 16:30 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 16:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Will Deacon <will.deacon@arm.com> - 2017-07-27 16:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Boqun Feng <boqun.feng@gmail.com> - 2017-07-27 16:50 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 17:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Will Deacon <will.deacon@arm.com> - 2017-07-26 11:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-26 17:50 +0200
              Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 12:20 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 15:00 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option Peter Zijlstra <peterz@infradead.org> - 2017-07-27 15:40 +0200
                Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 16:40 +0200

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