On Wed, Jul 17, 2013 at 07:43:01PM +0530, Raghavendra K T wrote:

On 07/17/2013 06:55 PM, Gleb Natapov wrote:

quoted

On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote:

quoted

On 07/17/2013 06:15 PM, Gleb Natapov wrote:

quoted

On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote:

quoted

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Instead of halt we started with a sleep hypercall in those
 versions. Changed to halt() once Avi suggested to reuse existing sleep.

If we use older hypercall with few changes like below:

kvm_pv_wait_for_kick_op(flags, vcpu, w->lock )
{
 // a0 reserved for flags
if (!w->lock)
return;
DEFINE_WAIT
...
end_wait
}

How would this help if NMI takes lock in critical section. The thing
that may happen is that lock_waiting->want may have NMI lock value, but
lock_waiting->lock will point to non NMI lock. Setting of want and lock
have to be atomic.

True. so we are here

        non NMI lock(a)
        w->lock = NULL;
        smp_wmb();
        w->want = want;
                               NMI
                         <---------------------
                          NMI lock(b)
                          w->lock = NULL;
                          smp_wmb();
                          w->want = want;
                          smp_wmb();
                          w->lock = lock;
                         ---------------------->
        smp_wmb();
        w->lock = lock;

so how about fixing like this?

again:
        w->lock = NULL;
        smp_wmb();
        w->want = want;
        smp_wmb();
        w->lock = lock;

if (!lock || w->want != want) goto again;

NMI can happen after the if() but before halt and the same situation
we are trying to prevent with IRQs will occur.

True, we can not fix that. I thought to fix the inconsistency of
lock,want pair.
But NMI could happen after the first OR condition also.
/me thinks again

lock_spinning() can check that it is called in nmi context and bail out.

Good point.
I think we can check for even irq context and bailout so that in irq
context we continue spinning instead of slowpath. no ?

That will happen much more often and irq context is no a problem anyway.

quoted

How often this will happens anyway.

I know NMIs occur frequently with watchdogs. or used by sysrq-trigger
etc.. But I am not an expert how frequent it is otherwise. But even
then if they do not use spinlock, we have no problem as already pointed.

I can measure with debugfs counter how often it happens.

When you run perf you will see a lot of NMIs, but those should not take
any locks.

--
			Gleb.