[xen staging] xen/locking: harmonize spinlocks and rwlocks regarding preemption

[patchbot@xxxxxxx]

commit c3453a23f7905d24f2404787543e26ec7d02301c
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Fri Nov 6 10:47:09 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Nov 6 10:47:09 2020 +0100

    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@xxxxxxxx>
    Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 xen/common/spinlock.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

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 )
     {
--
generated by git-patchbot for /home/xen/git/xen.git#staging