watchdog/softlockup: Remove obsolete check of last reported task

commit 9cf57731 ("watchdog/softlockup: Replace "watchdog/%u" threads
 with cpu_stop_work") ensures that the watchdog is reliably touched during
a task switch.

As a result the check for an unnoticed task switch is not longer needed.

Remove the relevant code, which effectively reverts commit b1a8de1f
("softlockup: make detector be aware of task switch of processes hogging
cpu")
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Ziljstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20191024114928.15377-2-pmladek@suse.com

kernel/watchdog.c

@@ -173,7 +173,6 @@ static DEFINE_PER_CPU(struct hrtimer, watchdog_hrtimer);
 static DEFINE_PER_CPU(bool, softlockup_touch_sync);
 static DEFINE_PER_CPU(bool, soft_watchdog_warn);
 static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
-static DEFINE_PER_CPU(struct task_struct *, softlockup_task_ptr_saved);
 static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved);
 static unsigned long soft_lockup_nmi_warn;
 
@@ -413,22 +412,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 			return HRTIMER_RESTART;
 
 		/* only warn once */
-		if (__this_cpu_read(soft_watchdog_warn) == true) {
-			/*
-			 * When multiple processes are causing softlockups the
-			 * softlockup detector only warns on the first one
-			 * because the code relies on a full quiet cycle to
-			 * re-arm.  The second process prevents the quiet cycle
-			 * and never gets reported.  Use task pointers to detect
-			 * this.
-			 */
-			if (__this_cpu_read(softlockup_task_ptr_saved) !=
-			    current) {
-				__this_cpu_write(soft_watchdog_warn, false);
-				__touch_watchdog();
-			}
+		if (__this_cpu_read(soft_watchdog_warn) == true)
 			return HRTIMER_RESTART;
-		}
 
 		if (softlockup_all_cpu_backtrace) {
 			/* Prevent multiple soft-lockup reports if one cpu is already
@@ -444,7 +429,6 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 		pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
 			smp_processor_id(), duration,
 			current->comm, task_pid_nr(current));
-		__this_cpu_write(softlockup_task_ptr_saved, current);
 		print_modules();
 		print_irqtrace_events(current);
 		if (regs)