From: Peng Hao <[email protected]>
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 <[email protected]>
---
arch/x86/kvm/xen.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
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;
@@ -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
On Fri, Oct 07, 2022, Hao Peng wrote:
> From: Peng Hao <[email protected]>
>
> 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.