From: Jan Beulich <jbeulich@suse.com>
Date: Wed, 25 Nov 2020 13:08:14 +0000 (+0100)
Subject: evtchn: double per-channel locking can't hit identical channels
X-Git-Tag: archive/raspbian/4.16.0+51-g0941d6cb-1+rpi1~2^2~42^2~1380
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=181f2c224ccd0a2900d6ae94ec390a546731f593;p=xen.git

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

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

diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index 54b2e2550e..dbfba62a49 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -326,23 +326,18 @@ static long evtchn_alloc_unbound(evtchn_alloc_unbound_t *alloc)
 
 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);
 }