[RFC][PATCH 3/3] x86/nmi: Perform a safe NMI stack trace on all CPUs

Steven Rostedt
2014-06-19T21:39:59Z

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From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

When trigger_all_cpu_backtrace() is called on x86, it will trigger an
NMI on each CPU and call show_regs(). But this can lead to a hard lock
up if the NMI comes in on another printk().

In order to avoid this, when the NMI triggers, it switches the printk
routine for that CPU to call a NMI safe printk function that records the
printk in a per_cpu trace_seq descriptor. After all NMIs have finished
recording its data, the trace_seqs are printed in a safe context.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
arch/x86/kernel/apic/hw_nmi.c | 66 ++++++++++++++++++++++++++++++++++++++++---
1 file changed, 62 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index c3fcb5de5083..6731604bb1cd 100644
--- a/arch/x86/kernel/apic/hw_nmi.c
+++ b/arch/x86/kernel/apic/hw_nmi.c
@@ -18,6 +18,7 @@
#include <linux/nmi.h>
#include <linux/module.h>
#include <linux/delay.h>
+#include <linux/trace_seq.h>

#ifdef CONFIG_HARDLOCKUP_DETECTOR
u64 hw_nmi_get_sample_period(int watchdog_thresh)
@@ -30,11 +31,30 @@ u64 hw_nmi_get_sample_period(int watchdog_thresh)
/* For reliability, we're prepared to waste bits here. */
static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;

+/* Safe printing in NMI context */
+static DEFINE_PER_CPU(struct trace_seq, nmi_print_seq);
+
/* "in progress" flag of arch_trigger_all_cpu_backtrace */
static unsigned long backtrace_flag;

+static void print_seq_line(struct trace_seq *s, int last, int pos)
+{
+ const char *buf = s->buffer + last;
+
+ /* Chop off the saved log level and update the length */
+ if (printk_get_level(buf)) {
+ buf += 2;
+ last += 2;
+ }
+
+ pr_emerg("%.*s", (pos - last) + 1, buf);
+}
+
void arch_trigger_all_cpu_backtrace(void)
{
+ struct trace_seq *s;
+ int len;
+ int cpu;
int i;

if (test_and_set_bit(0, &backtrace_flag))
@@ -44,6 +64,11 @@ void arch_trigger_all_cpu_backtrace(void)
*/
return;

+ for_each_possible_cpu(i) {
+ s = &per_cpu(nmi_print_seq, i);
+ trace_seq_init(s);
+ }
+
cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);

printk(KERN_INFO "sending NMI to all CPUs:\n");
@@ -56,8 +81,39 @@ void arch_trigger_all_cpu_backtrace(void)
mdelay(1);
}

+ for_each_possible_cpu(cpu) {
+ int last_i = 0;
+
+ s = &per_cpu(nmi_print_seq, cpu);
+ len = s->len;
+ if (!len)
+ continue;
+
+ /* Print line by line. */
+ for (i = 0; i < len; i++) {
+ if (s->buffer[i] == '\n') {
+ print_seq_line(s, last_i, i);
+ last_i = i + 1;
+ }
+ }
+ if (last_i < i - 1) {
+ print_seq_line(s, last_i, i);
+ pr_cont("\n");
+ }
+ }
+
clear_bit(0, &backtrace_flag);
smp_mb__after_atomic();
+ return true;
+}
+
+static int nmi_vprintk(const char *fmt, va_list args)
+{
+ struct trace_seq *s = this_cpu_ptr(&nmi_print_seq);
+ int len = s->len;
+
+ trace_seq_vprintf(s, fmt, args);
+ return s->len - len;
}

static int
@@ -68,12 +124,14 @@ arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
cpu = smp_processor_id();

if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
- static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
+ printk_func_t printk_func_save = this_cpu_read(printk_func);

- arch_spin_lock(&lock);
- printk(KERN_WARNING "NMI backtrace for cpu %d\n", cpu);
+ /* Replace printk to write into the NMI seq */
+ this_cpu_write(printk_func, nmi_vprintk);
+ printk("NMI backtrace for cpu %d\n", cpu);
show_regs(regs);
- arch_spin_unlock(&lock);
+ this_cpu_write(printk_func, printk_func_save);
+
cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
return NMI_HANDLED;
}
--
2.0.0.rc2