[PATCH v3] KVM: x86: Keep the lock order consistent

[PATCH v3] KVM: x86: Keep the lock order consistent

[Hao Peng]

From: Peng Hao <flyingpeng@tencent.com>

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 <flyingpeng@tencent.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
---
 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

Re: [PATCH v3] KVM: x86: Keep the lock order consistent

[Sean Christopherson]

On Tue, Nov 08, 2022, Hao Peng wrote:
> From: Peng Hao <flyingpeng@tencent.com>
> 
> 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 <flyingpeng@tencent.com>
> Reviewed-by: Sean Christopherson <seanjc@google.com>
> ---

Merged to kvm/queue, thanks!

https://lore.kernel.org/all/Y4lHxds8pvBhxXFX@google.com