On Tue, 9 Jan 2007 17:06:16 -0500 Mathieu Desnoyers <[EMAIL PROTECTED]> wrote:
> * Andrew Morton ([EMAIL PROTECTED]) wrote: > > On Mon, 8 Jan 2007 22:14:46 -0500 > > Mathieu Desnoyers <[EMAIL PROTECTED]> 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/