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Re: [PATCH v3 2/2] xen/evtchn: rework per event channel lock



On 02.11.20 14:52, Jan Beulich wrote:
On 02.11.2020 14:41, Jürgen Groß wrote:
On 20.10.20 11:28, Jan Beulich wrote:
On 16.10.2020 12:58, Juergen Gross wrote:
--- a/xen/arch/x86/pv/shim.c
+++ b/xen/arch/x86/pv/shim.c
@@ -660,11 +660,12 @@ void pv_shim_inject_evtchn(unsigned int port)
       if ( port_is_valid(guest, port) )
       {
           struct evtchn *chn = evtchn_from_port(guest, port);
-        unsigned long flags;
- spin_lock_irqsave(&chn->lock, flags);
-        evtchn_port_set_pending(guest, chn->notify_vcpu_id, chn);
-        spin_unlock_irqrestore(&chn->lock, flags);
+        if ( evtchn_read_trylock(chn) )
+        {
+            evtchn_port_set_pending(guest, chn->notify_vcpu_id, chn);
+            evtchn_read_unlock(chn);
+        }

Does this want some form of else, e.g. at least a printk_once()?

No, I don't think so.

This is just a race with the port_is_valid() test above where the
port is just being switched to invalid.

This may be such a race yes, but why do you think it _will_ be?

According to the outlined lock discipline there is no other
possibility (assuming that the lock discipline is honored).

I'll have a look whether I can add some ASSERT()s to catch any
lock discipline violation.


@@ -360,7 +352,7 @@ static long 
evtchn_bind_interdomain(evtchn_bind_interdomain_t *bind)
       if ( rc )
           goto out;
- flags = double_evtchn_lock(lchn, rchn);
+    double_evtchn_lock(lchn, rchn);

This introduces an unfortunate conflict with my conversion of
the per-domain event lock to an rw one: It acquires rd's lock
in read mode only, while the requirements here would not allow
doing so. (Same in evtchn_close() then.)

Is it a problem to use write mode for those cases?

"Problem" can have a wide range of meanings - it's not going to
be the end of the world, but I view any use of a write lock as
a problem when a read lock would suffice. This can still harm
parallelism.

Both cases are very rare ones in the life time of an event channel. I
don't think you'll ever be able to measure any performance impact from
switching these case to a write lock for any well behaved guest.


Juergen



 


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