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SubjectRe: [PATCH RFC V11 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor
On 07/23/2013 08:37 PM, Gleb Natapov wrote:
> On Mon, Jul 22, 2013 at 11:50:16AM +0530, Raghavendra K T wrote:
>> +static void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
[...]
>> +
>> + /*
>> + * halt until it's our turn and kicked. Note that we do safe halt
>> + * for irq enabled case to avoid hang when lock info is overwritten
>> + * in irq spinlock slowpath and no spurious interrupt occur to save us.
>> + */
>> + if (arch_irqs_disabled_flags(flags))
>> + halt();
>> + else
>> + safe_halt();
>> +
>> +out:
> So here now interrupts can be either disabled or enabled. Previous
> version disabled interrupts here, so are we sure it is safe to have them
> enabled at this point? I do not see any problem yet, will keep thinking.

If we enable interrupt here, then


>> + cpumask_clear_cpu(cpu, &waiting_cpus);

and if we start serving lock for an interrupt that came here,
cpumask clear and w->lock=null may not happen atomically.
if irq spinlock does not take slow path we would have non null value for
lock, but with no information in waitingcpu.

I am still thinking what would be problem with that.

>> + w->lock = NULL;
>> + local_irq_restore(flags);
>> + spin_time_accum_blocked(start);
>> +}
>> +PV_CALLEE_SAVE_REGS_THUNK(kvm_lock_spinning);
>> +
>> +/* Kick vcpu waiting on @lock->head to reach value @ticket */
>> +static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
>> +{
>> + int cpu;
>> +
>> + add_stats(RELEASED_SLOW, 1);
>> + for_each_cpu(cpu, &waiting_cpus) {
>> + const struct kvm_lock_waiting *w = &per_cpu(lock_waiting, cpu);
>> + if (ACCESS_ONCE(w->lock) == lock &&
>> + ACCESS_ONCE(w->want) == ticket) {
>> + add_stats(RELEASED_SLOW_KICKED, 1);
>> + kvm_kick_cpu(cpu);
> What about using NMI to wake sleepers? I think it was discussed, but
> forgot why it was dismissed.

I think I have missed that discussion. 'll go back and check. so what is
the idea here? we can easily wake up the halted vcpus that have
interrupt disabled?



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