[PATCH] arm64: percpu: Make this_cpu accessors pre-empt safe

[mark.rutland@arm.com (Mark Rutland)]

On Thu, Mar 19, 2015 at 04:00:09PM +0000, Will Deacon wrote:
> On Thu, Mar 19, 2015 at 03:44:36PM +0000, Mark Rutland wrote:
> > On Thu, Mar 19, 2015 at 02:52:31PM +0000, Steve Capper wrote:
> > > this_cpu operations were implemented for arm64 in:
> > >  5284e1b arm64: xchg: Implement cmpxchg_double
> > >  f97fc81 arm64: percpu: Implement this_cpu operations
> > > 
> > > Unfortunately, it is possible for pre-emption to take place between
> > > address generation and data access. This can lead to cases where data
> > > is being manipulated by this_cpu for a different CPU than it was
> > > called on. Which effectively breaks the spec.
> > > 
> > > This patch disables pre-emption for the this_cpu operations
> > > guaranteeing that address generation and data manipulation.
> > 
> > Shouldn't that last sentence end with "occur on the same CPU", or
> > something like that?
> > 
> > [...]
> > 
> > > +/*
> > > + * Modules aren't allowed to use preempt_enable_no_resched, and it is
> > > + * undef'ed. If we are unable to use preempt_enable_no_resched, then
> > > + * fallback to the standard preempt_enable.
> > > + */
> > > +#ifdef preempt_enable_no_resched
> > > +#define __pcp_preempt_enable()	preempt_enable_no_resched()
> > > +#else
> > > +#define __pcp_preempt_enable()	preempt_enable()
> > > +#endif /* preempt_enable_no_resched */
> > 
> > I think it would be worth mentioning in the comment why we want to use
> > preempt_enable_no_resched where possible (e.g. read-modify-cmpxchg
> > sequences where we want to have as few retries as possible).
> 
> Hmm, I'm not sure I agree with that. In the interest of throughput, I can
> understand that you want to minimise the retries but since preempt kernels
> are all about minimising latency then actually scheduling when a cmpxchg
> loop fail sounds pretty ideal to me.

I'm on about scheduling at the end of the read, before the cmpxchg. It's
basically asking for another thread to make the read stale (and hence
the cmpxchg is very likely to fail).

Scheduling after the cmpxchg is fine.

Mark.