4.4-stable review patch. If anyone has any objections, please let me know.
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From: Christoffer Dall christoffer.dall@linaro.org
[ Upstream commit 63e41226afc3f7a044b70325566fa86ac3142538 ]
When a VCPU blocks (WFI) and has programmed the vtimer, we program a soft timer to expire in the future to wake up the vcpu thread when appropriate. Because such as wake up involves a vcpu kick, and the timer expire function can get called from interrupt context, and the kick may sleep, we have to schedule the kick in the work function.
The work function currently has a warning that gets raised if it turns out that the timer shouldn't fire when it's run, which was added because the idea was that in that case the work should never have been cancelled.
However, it turns out that this whole thing is racy and we can get spurious warnings. The problem is that we clear the armed flag in the work function, which may run in parallel with the kvm_timer_unschedule->timer_disarm() call. This results in a possible situation where the timer_disarm() call does not call cancel_work_sync(), which effectively synchronizes the completion of the work function with running the VCPU. As a result, the VCPU thread proceeds before the work function completees, causing changes to the timer state such that kvm_timer_should_fire(vcpu) returns false in the work function.
All we do in the work function is to kick the VCPU, and an occasional rare extra kick never harmed anyone. Since the race above is extremely rare, we don't bother checking if the race happens but simply remove the check and the clearing of the armed flag from the work function.
Reported-by: Matthias Brugger mbrugger@suse.com Reviewed-by: Marc Zyngier marc.zyngier@arm.com Signed-off-by: Christoffer Dall christoffer.dall@linaro.org Signed-off-by: Marc Zyngier marc.zyngier@arm.com Signed-off-by: Sasha Levin alexander.levin@verizon.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- virt/kvm/arm/arch_timer.c | 3 --- 1 file changed, 3 deletions(-)
--- a/virt/kvm/arm/arch_timer.c +++ b/virt/kvm/arm/arch_timer.c @@ -84,9 +84,6 @@ static void kvm_timer_inject_irq_work(st struct kvm_vcpu *vcpu;
vcpu = container_of(work, struct kvm_vcpu, arch.timer_cpu.expired); - vcpu->arch.timer_cpu.armed = false; - - WARN_ON(!kvm_timer_should_fire(vcpu));
/* * If the vcpu is blocked we want to wake it up so that it will see