linux-kernel - Re: [PATCH RFC V11 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

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Date:	Wed, 24 Jul 2013 15:15:50 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	mingo@...hat.com, jeremy@...p.org, x86@...nel.org,
	konrad.wilk@...cle.com, hpa@...or.com, pbonzini@...hat.com,
	linux-doc@...r.kernel.org, habanero@...ux.vnet.ibm.com,
	xen-devel@...ts.xensource.com, peterz@...radead.org,
	mtosatti@...hat.com, stefano.stabellini@...citrix.com,
	andi@...stfloor.org, attilio.rao@...rix.com, ouyang@...pitt.edu,
	gregkh@...e.de, agraf@...e.de, chegu_vinod@...com,
	torvalds@...ux-foundation.org, avi.kivity@...il.com,
	tglx@...utronix.de, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, riel@...hat.com, drjones@...hat.com,
	virtualization@...ts.linux-foundation.org,
	srivatsa.vaddagiri@...il.com
Subject: Re: [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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