[PATCH 3/4] watchdog/hardlockup: Use printk_cpu_sync_get_irqsave() to serialize reporting

[Douglas Anderson]

If two CPUs end up reporting a hardlockup at the same time then their
logs could get interleaved which is hard to read.

The interleaving problem was especially bad with the "perf" hardlockup
detector where the locked up CPU is always the same as the running CPU
and we end up in show_regs(). show_regs() has no inherent
serialization so we could mix together two crawls if two hardlockups
happened at the same time (and if we didn't have
`sysctl_hardlockup_all_cpu_backtrace` set). With this change we'll
fully serialize hardlockups when using the "perf" hardlockup detector.

The interleaving problem was less bad with the "buddy" hardlockup
detector. With "buddy" we always end up calling
`trigger_single_cpu_backtrace(cpu)` on some CPU other than the running
one. trigger_single_cpu_backtrace() always at least serializes the
individual stack crawls because it eventually uses
printk_cpu_sync_get_irqsave(). Unfortunately the fact that
trigger_single_cpu_backtrace() eventually calls
printk_cpu_sync_get_irqsave() (on a different CPU) means that we have
to drop the "lock" before calling it and we can't fully serialize all
printouts associated with a given hardlockup. However, we still do get
the advantage of serializing the output of print_modules() and
print_irqtrace_events().

Aside from serializing hardlockups from each other, this change also
has the advantage of serializing hardlockups and softlockups from each
other if they happen to happen at the same time since they are both
using the same "lock".

Even though nobody is expected to hang while holding the lock
associated with printk_cpu_sync_get_irqsave(), out of an abundance of
caution, we don't call printk_cpu_sync_get_irqsave() until after we
print out about the hardlockup. This makes extra sure that, even if
printk_cpu_sync_get_irqsave() somehow never runs we at least print
that we saw the hardlockup. This is different than the choice made for
softlockup because hardlockup is really our last resort.

Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

 kernel/watchdog.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 526041a1100a..11f9577accca 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -151,6 +151,7 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 	 */
 	if (is_hardlockup(cpu)) {
 		unsigned int this_cpu = smp_processor_id();
+		unsigned long flags;
 
 		/* Only print hardlockups once. */
 		if (per_cpu(watchdog_hardlockup_warned, cpu))
@@ -165,7 +166,17 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 				return;
 		}
 
+		/*
+		 * NOTE: we call printk_cpu_sync_get_irqsave() after printing
+		 * the lockup message. While it would be nice to serialize
+		 * that printout, we really want to make sure that if some
+		 * other CPU somehow locked up while holding the lock associated
+		 * with printk_cpu_sync_get_irqsave() that we can still at least
+		 * get the message about the lockup out.
+		 */
 		pr_emerg("Watchdog detected hard LOCKUP on cpu %d\n", cpu);
+		printk_cpu_sync_get_irqsave(flags);
+
 		print_modules();
 		print_irqtrace_events(current);
 		if (cpu == this_cpu) {
@@ -173,7 +184,9 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 				show_regs(regs);
 			else
 				dump_stack();
+			printk_cpu_sync_put_irqrestore(flags);
 		} else {
+			printk_cpu_sync_put_irqrestore(flags);
 			trigger_single_cpu_backtrace(cpu);
 		}
 
-- 
2.43.0.472.g3155946c3a-goog