Re: [PATCH] local_t : Documentation

[Andrew Morton <akpm@osdl.org>]

On Tue, 9 Jan 2007 17:06:16 -0500
Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> * Andrew Morton (akpm@osdl.org) wrote:
> > On Mon, 8 Jan 2007 22:14:46 -0500
> > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> > 
> > > +* How to use local atomic operations
> > > +
> > > +#include <linux/percpu.h>
> > > +#include <asm/local.h>
> > > +
> > > +static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
> > > +
> > > +
> > > +* Counting
> > > +
> > > +In preemptible context, use get_cpu_var() and put_cpu_var() around local atomic
> > > +operations : it makes sure that preemption is disabled around write access to
> > > +the per cpu variable. For instance :
> > > +
> > > +	local_inc(&get_cpu_var(counters));
> > > +	put_cpu_var(counters);
> > 
> > Confused.  The whole point behind local_t is that we can do
> > atomic-wrt-interrupts inc and dec on them.
> > 
> > Consequently, as atomic-wrt-interrupts means atomic-wrt-preemption, there
> > is no need to do a preempt_disable() around local_inc() and local_dec().
> > 
> 
> Hi Andrew,
> 
> Not exactly : the increment operation is atomic, but not the selection of the
> local variable. local_inc(&__get_cpu_var()) implies the following sequence 
> of operations :
> 
> 1 - Get the variable copy corresponding to the currently running CPU.
> 2 - atomically increment the variable.
> 
> It would be wrong to be scheduled on another CPU between 1 and 2, because the
> atomic increment should only be done by the CPU "owner" of the local variable,
> as the local atomic increment is not atomic wrt other CPUs.
> 

doh.  I knew that.