* 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. Mathieu -- OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - 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/