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SubjectRe: [PATCH] Fixes for RCU handling of task_struct
Ingo Molnar <mingo@elte.hu> wrote:
>
> ...
>
> RCU signal handling: send signals RCU-read-locked instead of
> tasklist_lock read-locked. This is a scalability improvement on SMP and
> a preemption-latency improvement under PREEMPT_RCU.
>
> Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
> ...
> +static inline int get_task_struct_rcu(struct task_struct *t)
> +{
> + int oldusage;
> +
> + do {
> + oldusage = atomic_read(&t->usage);
> + if (oldusage == 0) {
> + return 0;
> + }
> + } while (cmpxchg(&t->usage.counter,
> + oldusage, oldusage + 1) != oldusage);
> + return 1;
> +}

arm (at least) does not implement cmpxchg.

I think Nick is working on patches which implement cmpxchg on all
architectures?
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