lkml.org 
[lkml]   [2015]   [Feb]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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



\
 
 \ /
  Last update: 2015-02-21 04:41    [W:0.391 / U:0.052 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site