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SubjectRe: [PATCH RFC nohz_full 6/7] nohz_full: Add full-system-idle state machine
On Thu, Jul 18, 2013 at 10:06:25PM -0700, Paul E. McKenney wrote:
> > Lets summarize the last sequence, the following happens ordered by time:
> >
> > CPU 0 CPU 1
> >
> > cmpxchg(&full_sysidle_state,
> > RCU_SYSIDLE_SHORT,
> > RCU_SYSIDLE_LONG);
> >
> > smp_mb() //cmpxchg
> >
> > atomic_read(rdtp(1)->dynticks_idle)
> >
> > //CPU 0 goes to sleep
> > //CPU 1 wakes up
> > atomic_inc(rdtp(1)->dynticks_idle)
> >
> > smp_mb()
> >
> > ACCESS_ONCE(full_sysidle_state)
> >
> >
> > Are you suggesting that because the CPU 1 executes its atomic_inc() _after_ (in terms
> > of absolute time) the atomic_read of CPU 0, the ordering settled in both sides guarantees
> > that the value read from CPU 1 is the one from the cmpxchg that precedes the atomic_read,
> > or FULL or FULL_NOTED that happen later.
> >
> > If so that's a big lesson for me.
>
> It is not absolute time that matters. Instead, it is the fact that
> CPU 0, when reading from ->dynticks_idle, read the old value before the
> atomic_inc(). Therefore, anything CPU 0 did before that memory barrier
> preceding CPU 0's read must come before anything CPU 1 did after that
> memory barrier following the atomic_inc(). For this to work, there
> must be some access to the same variable on each CPU.

Aren't we in the following situation?

CPU 0 CPU 1

STORE A STORE B
LOAD B LOAD A


If so and referring to your perfbook, this is an "ears to mouth" situation.
And it seems to describe there is no strong guarantee in that situation.

>
> Or, if you must think in terms of time, you need a separate independent
> timeline for each variable, with no direct mapping from one timeline to
> another, except resulting from memory-barrier interactions.
>
> Thanx, Paul
>


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