Re: [PATCH 05/10] nohz: New tick dependency mask

[Chris Metcalf]

On 07/24/2015 01:16 PM, Frederic Weisbecker wrote:
> On Fri, Jul 24, 2015 at 12:55:35PM -0400, Chris Metcalf wrote:
> > On 07/23/2015 12:42 PM, Frederic Weisbecker wrote:
> > > +unsigned long __tick_nohz_set_tick_dependency(enum tick_dependency_bit bit,
> > > +					      unsigned long *dep)
> > > +{
> > > +	unsigned long prev;
> > > +	unsigned long old = *dep;
> > > +	unsigned long mask = BIT_MASK(bit);
> > > +
> > > +	while ((prev = cmpxchg(dep, old, old | mask)) != old) {
> > > +		old = prev;
> > > +		cpu_relax();
> > > +	}
> > > +
> > > +	return prev;
> > > +}
> > Why not use set_bit() here?  It is suitably atomic.
> Because I don't want to send an IPI if the CPU already had bits set in
> the dependency.
>
> Ideally I need something like test_and_set_bit() but which returns the
> whole previous value and not just the previous value of the bit.

Ah, of course.  Peter, maybe we need atomic_or_return() as part
of your new atomic_or/_and/_xor series?  Certainly on tilegx, and
likely other architectures, we can do better than Frederic's
cmpxchg() loop.

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/