Re: [PATCH] Fixes for RCU handling of task_struct

[Andrew Morton <akpm@osdl.org>]

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?