From: Juergen Gross <jgross@suse.com>
Date: Fri, 6 Nov 2020 09:47:09 +0000 (+0100)
Subject: xen/locking: harmonize spinlocks and rwlocks regarding preemption
X-Git-Tag: archive/raspbian/4.16.0+51-g0941d6cb-1+rpi1~2^2~42^2~1429
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=c3453a23f7905d24f2404787543e26ec7d02301c;p=xen.git

xen/locking: harmonize spinlocks and rwlocks regarding preemption

Spinlocks and rwlocks behave differently in the try variants regarding
preemption: rwlocks are switching preemption off before testing the
lock, while spinlocks do so only after the first check.

Modify _spin_trylock() to disable preemption before testing the lock
to be held in order to be preemption-ready.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
---

diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
index b4aaf6bce6..f4eb50f030 100644
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -240,13 +240,16 @@ int _spin_trylock(spinlock_t *lock)
 {
     spinlock_tickets_t old, new;
 
+    preempt_disable();
     check_lock(&lock->debug, true);
     old = observe_lock(&lock->tickets);
     if ( old.head != old.tail )
+    {
+        preempt_enable();
         return 0;
+    }
     new = old;
     new.tail++;
-    preempt_disable();
     if ( cmpxchg(&lock->tickets.head_tail,
                  old.head_tail, new.head_tail) != old.head_tail )
     {