Re: [PATCH 2/3] KVM: Centralize slots_lock aquisition during KVM_CREATE_IRQCHIP

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(2010/12/16 18:11), Avi Kivity wrote:
On 12/15/2010 06:43 PM, Takuya Yoshikawa wrote:
From: Takuya Yoshikawa<yoshikawa.takuya@xxxxxxxxxxxxx>

Move slots_lock aquisition from kvm_ioapic_init() and kvm_create_pic()
to their caller.

As a result, x86's KVM_CREATE_IRQCHIP is now covered by a unified slots_lock
section, including kvm_setup_default_irq_routing().

I'm not sure about this...

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3300,6 +3300,7 @@ long kvm_arch_vm_ioctl(struct file *filp,
struct kvm_pic *vpic;

+ mutex_lock(&kvm->slots_lock);

here, the reader might wonder why we take slots_lock, since we aren't manipulating anything covered by the lock directly.

r = -EEXIST;
if (kvm->arch.vpic)
goto create_irqchip_unlock;
@@ -3308,10 +3309,8 @@ long kvm_arch_vm_ioctl(struct file *filp,
if (vpic) {
r = kvm_ioapic_init(kvm);
if (r) {
- mutex_lock(&kvm->slots_lock);

and here, the reader might wonder why we don't take slots_lock, which protects io_bus.

Maybe we ought to move slots_lock acquisition to kvm_io_bus_register() and friends.

So it will move the lock acquisition to the opposite ( callee ) side than mine.
  At first, I tried to do that, but there are so many ...

Anyway, your suggestion seems to be the best way if possible.

One question: how about kvm_io_bus_[read|write] ?

These are called from the emulator but I could not find where slots_lock
are held though I can see the comments

  "kvm_io_bus_[read|write] - called under kvm->slots_lock"

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