2022-10-31 07:40:00

by Hao Peng

Subject: [RESEND PATCH v2] kvm: x86: Keep the lock order consistent

From: Peng Hao <[email protected]>

Acquire SRCU before taking the gpc spinlock in wait_pending_event() so as
to be consistent with all other functions that acquire both locks. It's
not illegal to acquire SRCU inside a spinlock, nor is there deadlock
potential, but in general it's preferable to order locks from least
restrictive to most restrictive, e.g. if wait_pending_event() needed to
sleep for whatever reason, it could do so while holding SRCU, but would
need to drop the spinlock.

Thanks Sean Christopherson for the comment.

Signed-off-by: Peng Hao <[email protected]>
---
arch/x86/kvm/xen.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 2dae413bd62a..766e8a4ca3ea 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -964,8 +964,8 @@ static bool wait_pending_event(struct kvm_vcpu
*vcpu, int nr_ports,
bool ret = true;
int idx, i;

- read_lock_irqsave(&gpc->lock, flags);
idx = srcu_read_lock(&kvm->srcu);
+ read_lock_irqsave(&gpc->lock, flags);
if (!kvm_gfn_to_pfn_cache_check(kvm, gpc, gpc->gpa, PAGE_SIZE))
goto out_rcu;

@@ -986,8 +986,8 @@ static bool wait_pending_event(struct kvm_vcpu
*vcpu, int nr_ports,
}

out_rcu:
- srcu_read_unlock(&kvm->srcu, idx);
read_unlock_irqrestore(&gpc->lock, flags);
+ srcu_read_unlock(&kvm->srcu, idx);

return ret;
}
--
2.27.0

2022-11-03 23:48:25

by Sean Christopherson

Subject: Re: [RESEND PATCH v2] kvm: x86: Keep the lock order consistent

Nit, capitalize KVM in the shortlog, i.e. "KVM: x86:".

On Mon, Oct 31, 2022, Hao Peng wrote:
> From: Peng Hao <[email protected]>
>
> Acquire SRCU before taking the gpc spinlock in wait_pending_event() so as
> to be consistent with all other functions that acquire both locks. It's
> not illegal to acquire SRCU inside a spinlock, nor is there deadlock
> potential, but in general it's preferable to order locks from least
> restrictive to most restrictive, e.g. if wait_pending_event() needed to
> sleep for whatever reason, it could do so while holding SRCU, but would
> need to drop the spinlock.

Extra whitespace at the beginning of each line should be deleted.

> Thanks Sean Christopherson for the comment.
>
> Signed-off-by: Peng Hao <[email protected]>
> ---

Nits aside,

Reviewed-by: Sean Christopherson <[email protected]>