lib/nmi_backtrace: explicitly serialize banner and regs

Currently the nmi_backtrace is serialized against other CPUs because
the messages are sent to the NMI buffers. Once these buffers are
removed, only the dumped stack will be serialized against other CPUs
(via the printk_cpu_lock).

Also serialize the nmi_backtrace banner and regs using the
printk_cpu_lock so that per-CPU serialization will be preserved even
after the NMI buffers are removed.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210715193359.25946-2-john.ogness@linutronix.de

diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
index 8abe1870dba4e3762ad81949950b6076c489d913..dae233c5f597d4f0b41d221ae5be31abe0e7b00e 100644 (file)
--- a/lib/nmi_backtrace.c
+++ b/lib/nmi_backtrace.c
@@ -92,17 +92,24 @@ module_param(backtrace_idle, bool, 0644);
 bool nmi_cpu_backtrace(struct pt_regs *regs)
 {
        int cpu = smp_processor_id();
+       unsigned long flags;
 
        if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
                if (!READ_ONCE(backtrace_idle) && regs && cpu_in_idle(instruction_pointer(regs))) {
                        pr_warn("NMI backtrace for cpu %d skipped: idling at %pS\n",
                                cpu, (void *)instruction_pointer(regs));
                } else {
+                       /*
+                        * Allow nested NMI backtraces while serializing
+                        * against other CPUs.
+                        */
+                       printk_cpu_lock_irqsave(flags);
                        pr_warn("NMI backtrace for cpu %d\n", cpu);
                        if (regs)
                                show_regs(regs);
                        else
                                dump_stack();
+                       printk_cpu_unlock_irqrestore(flags);
                }
                cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
                return true;