[PATCH v2 4/5] powerpc/watchdog: Read TB close to where it is used

[Nicholas Piggin]

When taking watchdog actions, printing messages, comparing and
re-setting wd_smp_last_reset_tb, etc., read TB close to the point of use
and under wd_smp_lock or printing lock (if applicable).

This should keep timebase mostly monotonic with kernel log messages, and
could prevent (in theory) a laggy CPU updating wd_smp_last_reset_tb to
something a long way in the past, and causing other CPUs to appear to be
stuck.

These additional TB reads are all slowpath (lockup has been detected),
so performance does not matter.

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 arch/powerpc/kernel/watchdog.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index 0265d27340f1..2444cd10b61a 100644
--- a/arch/powerpc/kernel/watchdog.c
+++ b/arch/powerpc/kernel/watchdog.c
@@ -94,6 +94,10 @@ static u64 wd_smp_last_reset_tb;
  * Try to take the exclusive watchdog action / NMI IPI / printing lock.
  * wd_smp_lock must be held. If this fails, we should return and wait
  * for the watchdog to kick in again (or another CPU to trigger it).
+ *
+ * Importantly, if hardlockup_panic is set, wd_try_report failure should
+ * not delay the panic, because whichever other CPU is reporting will
+ * call panic.
  */
 static bool wd_try_report(void)
 {
@@ -153,7 +157,7 @@ static void wd_lockup_ipi(struct pt_regs *regs)
 	/* Do not panic from here because that can recurse into NMI IPI layer */
 }
 
-static bool set_cpu_stuck(int cpu, u64 tb)
+static bool set_cpu_stuck(int cpu)
 {
 	cpumask_set_cpu(cpu, &wd_smp_cpus_stuck);
 	cpumask_clear_cpu(cpu, &wd_smp_cpus_pending);
@@ -162,7 +166,7 @@ static bool set_cpu_stuck(int cpu, u64 tb)
 	 */
 	smp_mb();
 	if (cpumask_empty(&wd_smp_cpus_pending)) {
-		wd_smp_last_reset_tb = tb;
+		wd_smp_last_reset_tb = get_tb();
 		cpumask_andnot(&wd_smp_cpus_pending,
 				&wd_cpus_enabled,
 				&wd_smp_cpus_stuck);
@@ -171,14 +175,16 @@ static bool set_cpu_stuck(int cpu, u64 tb)
 	return false;
 }
 
-static void watchdog_smp_panic(int cpu, u64 tb)
+static void watchdog_smp_panic(int cpu)
 {
 	static cpumask_t wd_smp_cpus_ipi; // protected by reporting
 	unsigned long flags;
+	u64 tb;
 	int c;
 
 	wd_smp_lock(&flags);
 	/* Double check some things under lock */
+	tb = get_tb();
 	if ((s64)(tb - wd_smp_last_reset_tb) < (s64)wd_smp_panic_timeout_tb)
 		goto out;
 	if (cpumask_test_cpu(cpu, &wd_smp_cpus_pending))
@@ -192,7 +198,7 @@ static void watchdog_smp_panic(int cpu, u64 tb)
 			continue; // should not happen
 
 		__cpumask_set_cpu(c, &wd_smp_cpus_ipi);
-		if (set_cpu_stuck(c, tb))
+		if (set_cpu_stuck(c))
 			break;
 	}
 	if (cpumask_empty(&wd_smp_cpus_ipi)) {
@@ -232,7 +238,7 @@ static void watchdog_smp_panic(int cpu, u64 tb)
 	wd_smp_unlock(&flags);
 }
 
-static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
+static void wd_smp_clear_cpu_pending(int cpu)
 {
 	if (!cpumask_test_cpu(cpu, &wd_smp_cpus_pending)) {
 		if (unlikely(cpumask_test_cpu(cpu, &wd_smp_cpus_stuck))) {
@@ -240,7 +246,7 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
 			unsigned long flags;
 
 			pr_emerg("CPU %d became unstuck TB:%lld\n",
-				 cpu, tb);
+				 cpu, get_tb());
 			print_irqtrace_events(current);
 			if (regs)
 				show_regs(regs);
@@ -301,7 +307,7 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
 		 */
 		wd_smp_lock(&flags);
 		if (cpumask_empty(&wd_smp_cpus_pending)) {
-			wd_smp_last_reset_tb = tb;
+			wd_smp_last_reset_tb = get_tb();
 			cpumask_andnot(&wd_smp_cpus_pending,
 					&wd_cpus_enabled,
 					&wd_smp_cpus_stuck);
@@ -316,10 +322,10 @@ static void watchdog_timer_interrupt(int cpu)
 
 	per_cpu(wd_timer_tb, cpu) = tb;
 
-	wd_smp_clear_cpu_pending(cpu, tb);
+	wd_smp_clear_cpu_pending(cpu);
 
 	if ((s64)(tb - wd_smp_last_reset_tb) >= (s64)wd_smp_panic_timeout_tb)
-		watchdog_smp_panic(cpu, tb);
+		watchdog_smp_panic(cpu);
 }
 
 DEFINE_INTERRUPT_HANDLER_NMI(soft_nmi_interrupt)
@@ -356,7 +362,7 @@ DEFINE_INTERRUPT_HANDLER_NMI(soft_nmi_interrupt)
 			return 0;
 		}
 
-		set_cpu_stuck(cpu, tb);
+		set_cpu_stuck(cpu);
 
 		wd_smp_unlock(&flags);
 
@@ -417,7 +423,7 @@ void arch_touch_nmi_watchdog(void)
 	tb = get_tb();
 	if (tb - per_cpu(wd_timer_tb, cpu) >= ticks) {
 		per_cpu(wd_timer_tb, cpu) = tb;
-		wd_smp_clear_cpu_pending(cpu, tb);
+		wd_smp_clear_cpu_pending(cpu);
 	}
 }
 EXPORT_SYMBOL(arch_touch_nmi_watchdog);
@@ -475,7 +481,7 @@ static void stop_watchdog(void *arg)
 	cpumask_clear_cpu(cpu, &wd_cpus_enabled);
 	wd_smp_unlock(&flags);
 
-	wd_smp_clear_cpu_pending(cpu, get_tb());
+	wd_smp_clear_cpu_pending(cpu);
 }
 
 static int stop_watchdog_on_cpu(unsigned int cpu)
-- 
2.23.0