Messages in this thread | | | Date | Tue, 9 Jan 2007 17:06:16 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] local_t : Documentation |
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* 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.
Mathieu
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