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    SubjectRe: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles
    On Fri, Feb 20, 2015 at 07:28:16PM +0100, Manfred Spraul wrote:
    > Hi Oleg,
    >
    > my example was bad, let's continue with your example.
    >
    > And: If sem_lock() needs another smp_xmb(), then we must add it:
    > Some apps do not have a user space hot path, i.e. it seems that on
    > some setups, we have millions of calls per second.
    > If there is a race, then it will happen.
    >
    > I've tried to merge your example:
    > >
    > > int X = 0, Y = 0;
    > >
    > > void func(void)
    > > {
    > > bool ll = rand();
    > >
    > > if (ll) {
    > > spin_lock(&local);
    > > if (!spin_is_locked(&global))
    > > goto done;
    > > spin_unlock(&local);
    > > }
    > > ll = false;
    > > spin_lock(&global);
    > > spin_unlock_wait(&local);
    > > done:
    > > smp_rmb(); <<<<<<<<<<<<<<<
    > > BUG_ON(X != Y);
    > >
    > > ++X; ++Y;
    > >
    > > if (ll)
    > > spin_unlock(&local);
    > > else
    > > spin_unlock(&global);
    > > }
    > I agree, we need the smp_rmb().
    > I'll write a patch.
    >
    > >We need the full barrier to serialize STORE's as well, but probably we can
    > >rely on control dependancy and thus we only need rmb().
    > Do we need a full barrier or not?
    >
    > I don't manage to create a proper line of reasoning.

    This has to be one of the more bizarre forms of Dekker's algorithm
    that I have seen. ;-)

    I am going to have to put this through one of the tools...

    Thanx, Paul



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