Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

[Paul E. McKenney]

On Fri, Feb 20, 2015 at 07:26:21PM -0800, Paul E. McKenney wrote:
> 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...

And this was just me getting confused by memories of old versions of
the code.  This will work given current mainline code.  Please accept
my apologies for the noise.

And yes, you do need the smp_rmb() to ensure that the BUG_ON() happens
after the other guy releases his spinlock.

							Thanx, Paul

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