Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support forlinux guests running on KVM hypervisor
From: Raghavendra K T
Date: Wed Jul 17 2013 - 10:29:28 EST
On 07/17/2013 06:55 PM, Gleb Natapov wrote:
On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote:
On 07/17/2013 06:15 PM, Gleb Natapov wrote:
On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote:
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 ?
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.
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