rcu: Allow up to five minutes expedited RCU CPU stall-warning timeouts

The maximum value of RCU CPU stall-warning timeouts has historically been
five minutes (300 seconds).  However, the recently introduced expedited
RCU CPU stall-warning timeout is instead limited to 21 seconds.  This
causes problems for CI/fuzzing services such as syzkaller by obscuring
the issue in question with expedited RCU CPU stall-warning timeout splats.

This commit therefore sets the RCU_EXP_CPU_STALL_TIMEOUT Kconfig options
upper bound to 300000 milliseconds, which is 300 seconds (AKA 5 minutes).

[ paulmck: Apply feedback from Hillf Danton. ]
[ paulmck: Apply feedback from Geert Uytterhoeven. ]
Reported-by: Dave Chinner <david@fromorbit.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/Kconfig.debug

@@ -82,7 +82,7 @@ config RCU_CPU_STALL_TIMEOUT
 config RCU_EXP_CPU_STALL_TIMEOUT
 	int "Expedited RCU CPU stall timeout in milliseconds"
 	depends on RCU_STALL_COMMON
-	range 0 21000
+	range 0 300000
 	default 0
 	help
 	  If a given expedited RCU grace period extends more than the

kernel/rcu/tree_stall.h

@@ -39,7 +39,7 @@ int rcu_exp_jiffies_till_stall_check(void)
 	// CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
 	// The minimum clamped value is "2UL", because at least one full
 	// tick has to be guaranteed.
-	till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 21UL * HZ);
+	till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 300UL * HZ);
 
 	if (cpu_stall_timeout && jiffies_to_msecs(till_stall_check) != cpu_stall_timeout)
 		WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));