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KVM: s390: Fix hang VCPU hang/loop regression

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commit 785dbef407 ("KVM: s390: optimize round trip time in request
handling") introduced a regression. This regression was seen with
CPU hotplug in the guest and switching between 1 or 2 CPUs. This will
set/reset the IBS control via synced request.

Whenever we make a synced request, we first set the vcpu->requests
bit and then block the vcpu. The handler, on the other hand, unblocks
itself, processes vcpu->requests (by clearing them) and unblocks itself
once again.

Now, if the requester sleeps between setting of vcpu->requests and
blocking, the handler will clear the vcpu->requests bit and try to
unblock itself (although no bit is set). When the requester wakes up,
it blocks the VCPU and we have a blocked VCPU without requests.

Solution is to always unset the block bit.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Fixes: 785dbef407 ("KVM: s390: optimize round trip time in request handling")
This commit is contained in:
Christian Borntraeger 2015-07-28 15:03:05 +02:00
parent 0da029ed7e
commit 586b7ccdb7
1 changed files with 2 additions and 2 deletions
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4
arch/s390/kvm/kvm-s390.c
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@ -1742,10 +1742,10 @@ static bool ibs_enabled(struct kvm_vcpu *vcpu)
static int kvm_s390_handle_requests(struct kvm_vcpu *vcpu)
{
if (!vcpu->requests)
return 0;
retry:
kvm_s390_vcpu_request_handled(vcpu);
if (!vcpu->requests)
return 0;
/*
* We use MMU_RELOAD just to re-arm the ipte notifier for the
* guest prefix page. gmap_ipte_notify will wait on the ptl lock.