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

[patchbot@xxxxxxx]

commit 181f2c224ccd0a2900d6ae94ec390a546731f593
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Nov 25 14:08:14 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Nov 25 14:08:14 2020 +0100

    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@xxxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 xen/common/event_channel.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

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);
 }
 
--
generated by git-patchbot for /home/xen/git/xen.git#master