[PATCH] evtchn: double per-channel locking can't hit identical channels

[PATCH] evtchn: double per-channel locking can't hit identical channels

[Jan Beulich]

Inter-domain channels can't possibly be bound to themselves, there's
always a 2nd channel involved, even when this is a loopback into the
same domain. As a result we can drop one conditional each from the two
involved functions.

With this, the number of evtchn_write_lock() invocations can also be
shrunk by half, swapping the two incoming function arguments instead.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -326,23 +326,18 @@ static long evtchn_alloc_unbound(evtchn_
 
 static void double_evtchn_lock(struct evtchn *lchn, struct evtchn *rchn)
 {
-    if ( lchn <= rchn )
-    {
-        evtchn_write_lock(lchn);
-        if ( lchn != rchn )
-            evtchn_write_lock(rchn);
-    }
-    else
-    {
-        evtchn_write_lock(rchn);
-        evtchn_write_lock(lchn);
-    }
+    ASSERT(lchn != rchn);
+
+    if ( lchn > rchn )
+        SWAP(lchn, rchn);
+
+    evtchn_write_lock(lchn);
+    evtchn_write_lock(rchn);
 }
 
 static void double_evtchn_unlock(struct evtchn *lchn, struct evtchn *rchn)
 {
-    if ( lchn != rchn )
-        evtchn_write_unlock(lchn);
+    evtchn_write_unlock(lchn);
     evtchn_write_unlock(rchn);
 }
 

Re: [PATCH] evtchn: double per-channel locking can't hit identical channels

[Jürgen Groß]

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On 24.11.20 18:03, Jan Beulich wrote:
> Inter-domain channels can't possibly be bound to themselves, there's
> always a 2nd channel involved, even when this is a loopback into the
> same domain. As a result we can drop one conditional each from the two
> involved functions.
> 
> With this, the number of evtchn_write_lock() invocations can also be
> shrunk by half, swapping the two incoming function arguments instead.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen

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Re: [PATCH] evtchn: double per-channel locking can't hit identical channels

[Julien Grall]

Hi Jan,

On 24/11/2020 17:03, Jan Beulich wrote:
> Inter-domain channels can't possibly be bound to themselves, there's
> always a 2nd channel involved, even when this is a loopback into the
> same domain. As a result we can drop one conditional each from the two
> involved functions.
> 
> With this, the number of evtchn_write_lock() invocations can also be
> shrunk by half, swapping the two incoming function arguments instead.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Julien Grall <jgrall@amazon.com>

Cheers,

-- 
Julien Grall