kvm: x86: Keep the lock order consistent

Commit Message

Hao Peng Oct. 7, 2022, 3:56 p.m. UTC

From: Peng Hao <[e-mail]>

srcu read side in critical section may sleep, so it should precede
the read lock, while other paths such as kvm_xen_set_evtchn_fast
execute srcu_read_lock before acquiring the read lock.

Signed-off-by: Peng Hao <[e-mail]>
---
 arch/x86/kvm/xen.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)


@@ -987,9 +987,8 @@ 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

Comments

Sean Christopherson Oct. 7, 2022, 5:28 p.m. UTC | #1

On Fri, Oct 07, 2022, Hao Peng wrote:
> From: Peng Hao <[e-mail]>
> 
> srcu read side in critical section may sleep, so it should precede
> the read lock,

I agree with the patch, but not necessarily with this statement.  The above
implies that it's not safe to acquire SRCU while in a non-sleepable context,
which is incorrect.  E.g. at first I thought the above implied there is an
incorrect sleep buried in this code.

> while other paths such as kvm_xen_set_evtchn_fast

Please put parantheses after function names, e.g. kvm_xen_set_evtchn_fast()
and srcu_read_lock().

> execute srcu_read_lock before acquiring the read lock.

How about this for a changelog?

  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.

Patch

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 280cb5dc7341..fa6e54b13afb 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -965,8 +965,8 @@  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;