On 11/08/2010 01:21 PM, Jan Kiszka wrote:
Ack and mask notifiers typically call back into kvm_set_irq, thus may
iterate over all VCPUs of a VM. Better keep this path preemptible to
prevent that user-space can massivle influence scheduling latencies. Use
sleepable RCU for the protection of irq_routing and the notfier lists.


What about preemptible RCU, now in mainline?

/*
 * Tree-preemptable RCU implementation for rcu_read_lock().
 * Just increment ->rcu_read_lock_nesting, shared state will be updated
 * if we block.
 */
void __rcu_read_lock(void)
{
    current->rcu_read_lock_nesting++;
    barrier();  /* needed if we ever invoke rcu_read_lock in rcutree.c */
}
EXPORT_SYMBOL_GPL(__rcu_read_lock);

(wow, quite clever in its simplicity)

--
error compiling committee.c: too many arguments to function

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