kernel/smp: Make csdlock_debug= resettable

It is currently possible to set the csdlock_debug_enabled static
branch, but not to reset it.  This is an issue when several different
entities supply kernel boot parameters and also for kernels built with
CONFIG_CSD_LOCK_WAIT_DEBUG_DEFAULT=y.

Therefore, make the csdlock_debug=0 kernel boot parameter turn off
debugging.  Last one wins!
Reported-by: Jes Sorensen <Jes.Sorensen@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230321005516.50558-4-paulmck@kernel.org

Documentation/admin-guide/kernel-parameters.txt

@@ -889,12 +889,13 @@
 	cs89x0_media=	[HW,NET]
 			Format: { rj45 | aui | bnc }
 
-	csdlock_debug=	[KNL] Enable debug add-ons of cross-CPU function call
-			handling. When switched on, additional debug data is
-			printed to the console in case a hanging CPU is
-			detected, and that CPU is pinged again in order to try
-			to resolve the hang situation.  The default value of
-			this option depends on the CSD_LOCK_WAIT_DEBUG_DEFAULT
+	csdlock_debug=	[KNL] Enable or disable debug add-ons of cross-CPU
+			function call handling. When switched on,
+			additional debug data is printed to the console
+			in case a hanging CPU is detected, and that
+			CPU is pinged again in order to try to resolve
+			the hang situation.  The default value of this
+			option depends on the CSD_LOCK_WAIT_DEBUG_DEFAULT
 			Kconfig option.
 
 	dasd=		[HW,NET]

kernel/smp.c

@@ -116,11 +116,16 @@ static DEFINE_STATIC_KEY_MAYBE(CONFIG_CSD_LOCK_WAIT_DEBUG_DEFAULT, csdlock_debug
  */
 static int __init csdlock_debug(char *str)
 {
+	int ret;
 	unsigned int val = 0;
 
-	get_option(&str, &val);
-	if (val)
-		static_branch_enable(&csdlock_debug_enabled);
+	ret = get_option(&str, &val);
+	if (ret) {
+		if (val)
+			static_branch_enable(&csdlock_debug_enabled);
+		else
+			static_branch_disable(&csdlock_debug_enabled);
+	}
 
 	return 1;
 }