powerpc/xmon: Fix invocation inside lock region

[ Upstream commit 8d4a862276a9c30a269d368d324fb56529e6d5fd ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index a5938fa..f752f77 100644 (file)
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -78,6 +78,9 @@ 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);
 
@@ -357,7 +360,6 @@ 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
@@ -366,11 +368,11 @@ 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 */
 }
@@ -3472,6 +3474,14 @@ 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;