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Re: [PATCH] local_t : Documentation
- From: Andrew Morton <akpm at osdl dot org>
- To: Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>
- Cc: Pavel Machek <pavel at suse dot cz>, linux-kernel at vger dot kernel dot org, Ingo Molnar <mingo at redhat dot com>, Greg Kroah-Hartman <gregkh at suse dot de>, Christoph Hellwig <hch at infradead dot org>, ltt-dev at shafik dot org, systemtap at sources dot redhat dot com, Douglas Niehaus <niehaus at eecs dot ku dot edu>, "Martin J. Bligh" <mbligh at mbligh dot org>, Thomas Gleixner <tglx at linutronix dot de>
- Date: Tue, 9 Jan 2007 14:11:12 -0800
- Subject: Re: [PATCH] local_t : Documentation
- References: <20061221001545.GP28643@Krystal> <20061223093358.GF3960@ucw.cz> <20070109031446.GA29426@Krystal> <20070109130110.8934c29f.akpm@osdl.org> <20070109220616.GA30535@Krystal>
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.