xen: make xen_qlock_wait() nestable

xen_qlock_wait() isn't safe for nested calls due to interrupts. A call
of xen_qlock_kick() might be ignored in case a deeper nesting level
was active right before the call of xen_poll_irq():

CPU 1:                                   CPU 2:
spin_lock(lock1)
                                         spin_lock(lock1)
                                         -> xen_qlock_wait()
                                            -> xen_clear_irq_pending()
                                            Interrupt happens
spin_unlock(lock1)
-> xen_qlock_kick(CPU 2)
spin_lock_irqsave(lock2)
                                         spin_lock_irqsave(lock2)
                                         -> xen_qlock_wait()
                                            -> xen_clear_irq_pending()
                                               clears kick for lock1
                                            -> xen_poll_irq()
spin_unlock_irq_restore(lock2)
-> xen_qlock_kick(CPU 2)
                                            wakes up
                                         spin_unlock_irq_restore(lock2)
                                         IRET
                                           resumes in xen_qlock_wait()
                                           -> xen_poll_irq()
                                           never wakes up

The solution is to disable interrupts in xen_qlock_wait() and not to
poll for the irq in case xen_qlock_wait() is called in nmi context.

Cc: stable@vger.kernel.org
Cc: Waiman.Long@hp.com
Cc: peterz@infradead.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/spinlock.c

@@ -39,29 +39,25 @@ static void xen_qlock_kick(int cpu)
  */
 static void xen_qlock_wait(u8 *byte, u8 val)
 {
+	unsigned long flags;
 	int irq = __this_cpu_read(lock_kicker_irq);
 
 	/* If kicker interrupts not initialized yet, just spin */
-	if (irq == -1)
+	if (irq == -1 || in_nmi())
 		return;
 
-	/* If irq pending already clear it and return. */
+	/* Guard against reentry. */
+	local_irq_save(flags);
+
+	/* If irq pending already clear it. */
 	if (xen_test_irq_pending(irq)) {
 		xen_clear_irq_pending(irq);
-		return;
+	} else if (READ_ONCE(*byte) == val) {
+		/* Block until irq becomes pending (or a spurious wakeup) */
+		xen_poll_irq(irq);
 	}
 
-	if (READ_ONCE(*byte) != val)
-		return;
-
-	/*
-	 * If an interrupt happens here, it will leave the wakeup irq
-	 * pending, which will cause xen_poll_irq() to return
-	 * immediately.
-	 */
-
-	/* Block until irq becomes pending (or perhaps a spurious wakeup) */
-	xen_poll_irq(irq);
+	local_irq_restore(flags);
 }
 
 static irqreturn_t dummy_handler(int irq, void *dev_id)