powerpc/xmon: Fix invocation inside lock region

Currently xmon needs to get devtree_lock (through rtas_token()) during its
invocation (at crash time). If there is a crash while devtree_lock is being
held, then xmon tries to get the lock but spins forever and never get into
the interactive debugger, as in the following case:

	int *ptr = NULL;
	raw_spin_lock_irqsave(&devtree_lock, flags);
	*ptr = 0xdeadbeef;

This patch avoids calling rtas_token(), thus trying to get the same lock,
at crash time. This new mechanism proposes getting the token at
initialization time (xmon_init()) and just consuming it at crash time.

This would allow xmon to be possible invoked independent of devtree_lock
being held or not.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/xmon/xmon.c

@@ -75,6 +75,9 @@ static int xmon_gate;
 #define xmon_owner 0
 #endif /* CONFIG_SMP */
 
+#ifdef CONFIG_PPC_PSERIES
+static int set_indicator_token = RTAS_UNKNOWN_SERVICE;
+#endif
 static unsigned long in_xmon __read_mostly = 0;
 static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT);
 
@@ -358,7 +361,6 @@ static inline void disable_surveillance(void)
 #ifdef CONFIG_PPC_PSERIES
 	/* Since this can't be a module, args should end up below 4GB. */
 	static struct rtas_args args;
-	int token;
 
 	/*
 	 * At this point we have got all the cpus we can into
@@ -367,11 +369,11 @@ static inline void disable_surveillance(void)
 	 * If we did try to take rtas.lock there would be a
 	 * real possibility of deadlock.
 	 */
-	token = rtas_token("set-indicator");
-	if (token == RTAS_UNKNOWN_SERVICE)
+	if (set_indicator_token == RTAS_UNKNOWN_SERVICE)
 		return;
 
-	rtas_call_unlocked(&args, token, 3, 1, NULL, SURVEILLANCE_TOKEN, 0, 0);
+	rtas_call_unlocked(&args, set_indicator_token, 3, 1, NULL,
+			   SURVEILLANCE_TOKEN, 0, 0);
 
 #endif /* CONFIG_PPC_PSERIES */
 }
@@ -3688,6 +3690,14 @@ static void xmon_init(int enable)
 		__debugger_iabr_match = xmon_iabr_match;
 		__debugger_break_match = xmon_break_match;
 		__debugger_fault_handler = xmon_fault_handler;
+
+#ifdef CONFIG_PPC_PSERIES
+		/*
+		 * Get the token here to avoid trying to get a lock
+		 * during the crash, causing a deadlock.
+		 */
+		set_indicator_token = rtas_token("set-indicator");
+#endif
 	} else {
 		__debugger = NULL;
 		__debugger_ipi = NULL;