Re: [BUG][tip/master] kernel panic while locking selftest at qspinlock_paravirt.h:137!

[Waiman Long]

On 07/10/2015 08:32 PM, Masami Hiramatsu wrote:
> On 2015/07/10 23:28, Peter Zijlstra wrote:
> > On Fri, Jul 10, 2015 at 03:57:46PM +0200, Ingo Molnar wrote:
> > > * Peter Zijlstra<peterz@xxxxxxxxxxxxx>  wrote:
> > > > Do we want to make double unlock non-fatal unconditionally?
> > > No, just don't BUG() out, don't crash the system - generate a warning?
> > So that would be a yes..
> >
> > Something like so then? Won't this generate a splat on that locking self
> > test then? And upset people?
> Hmm, yes, this still noisy...
> Can't we avoid double-unlock completely? it seems that this warning can
> happen randomly, which means pv-spinlock randomly broken, doesn't it?

It shouldn't randomly happen. The message should be printed at the first 
instance of double-unlock. If that is not case, there may be some 
problem in the code.

Anyway, I have an alternative fix that should better capture the problem:

-------------------------------
diff --git a/kernel/locking/qspinlock_paravirt.h 
b/kernel/locking/qspinlock_paravirt.h
index 04ab181..92fc54f 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -286,15 +286,24 @@ __visible void __pv_queued_spin_unlock(struct 
qspinlock *lock)
 {
     struct __qspinlock *l = (void *)lock;
     struct pv_node *node;
+    u8 lockval = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);

     /*
      * We must not unlock if SLOW, because in that case we must first
      * unhash. Otherwise it would be possible to have multiple @lock
      * entries, which would be BAD.
      */
-    if (likely(cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL))
+    if (likely(lockval == _Q_LOCKED_VAL))
         return;

+    if (unlikely(lockval != _Q_SLOW_VAL)) {
+        printk(KERN_WARNING
+               "pvqspinlock: lock 0x%lx has corrupted value 0x%x!\n",
+               (unsigned long)lock, atomic_read(&lock->val));
+        WARN_ON_ONCE(1);
+        return;
+    }
+
     /*
      * Since the above failed to release, this must be the SLOW path.
      * Therefore start by looking up the blocked node and unhashing it.

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