Commit a9edc880 authored by Steven Rostedt (Red Hat)'s avatar Steven Rostedt (Red Hat) Committed by Steven Rostedt
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x86/nmi: Perform a safe NMI stack trace on all CPUs

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 seq_buf descriptor. After all NMIs have finished
recording its data, the seq_bufs are printed in a safe context.


Tested-by: default avatarJiri Kosina <>
Acked-by: default avatarJiri Kosina <>
Acked-by: default avatarPaul E. McKenney <>
Reviewed-by: default avatarPetr Mladek <>
Signed-off-by: default avatarSteven Rostedt <>
parent afdc34a3
......@@ -18,6 +18,7 @@
#include <linux/nmi.h>
#include <linux/module.h>
#include <linux/delay.h>
#include <linux/seq_buf.h>
u64 hw_nmi_get_sample_period(int watchdog_thresh)
......@@ -29,14 +30,35 @@ u64 hw_nmi_get_sample_period(int watchdog_thresh)
#ifdef arch_trigger_all_cpu_backtrace
/* For reliability, we're prepared to waste bits here. */
static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
static cpumask_var_t printtrace_mask;
#define NMI_BUF_SIZE 4096
struct nmi_seq_buf {
unsigned char buffer[NMI_BUF_SIZE];
struct seq_buf seq;
/* Safe printing in NMI context */
static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
/* "in progress" flag of arch_trigger_all_cpu_backtrace */
static unsigned long backtrace_flag;
static void print_seq_line(struct nmi_seq_buf *s, int start, int end)
const char *buf = s->buffer + start;
printk("%.*s", (end - start) + 1, buf);
void arch_trigger_all_cpu_backtrace(bool include_self)
struct nmi_seq_buf *s;
int len;
int cpu;
int i;
int cpu = get_cpu();
int this_cpu = get_cpu();
if (test_and_set_bit(0, &backtrace_flag)) {
......@@ -49,7 +71,17 @@ void arch_trigger_all_cpu_backtrace(bool include_self)
cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
if (!include_self)
cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_mask));
cpumask_copy(printtrace_mask, to_cpumask(backtrace_mask));
* Set up per_cpu seq_buf buffers that the NMIs running on the other
* CPUs will write to.
for_each_cpu(cpu, to_cpumask(backtrace_mask)) {
s = &per_cpu(nmi_print_seq, cpu);
seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
if (!cpumask_empty(to_cpumask(backtrace_mask))) {
pr_info("sending NMI to %s CPUs:\n",
......@@ -65,11 +97,58 @@ void arch_trigger_all_cpu_backtrace(bool include_self)
* Now that all the NMIs have triggered, we can dump out their
* back traces safely to the console.
for_each_cpu(cpu, printtrace_mask) {
int last_i = 0;
s = &per_cpu(nmi_print_seq, cpu);
len = seq_buf_used(&s->seq);
if (!len)
/* 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;
/* Check if there was a partial line. */
if (last_i < len) {
print_seq_line(s, last_i, len - 1);
clear_bit(0, &backtrace_flag);
* It is not safe to call printk() directly from NMI handlers.
* It may be fine if the NMI detected a lock up and we have no choice
* but to do so, but doing a NMI on all other CPUs to get a back trace
* can be done with a sysrq-l. We don't want that to lock up, which
* can happen if the NMI interrupts a printk in progress.
* Instead, we redirect the vprintk() to this nmi_vprintk() that writes
* the content into a per cpu seq_buf buffer. Then when the NMIs are
* all done, we can safely dump the contents of the seq_buf to a printk()
* from a non NMI context.
static int nmi_vprintk(const char *fmt, va_list args)
struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
unsigned int len = seq_buf_used(&s->seq);
seq_buf_vprintf(&s->seq, fmt, args);
return seq_buf_used(&s->seq) - len;
static int
arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
......@@ -78,12 +157,14 @@ 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);
/* Replace printk to write into the NMI seq */
this_cpu_write(printk_func, nmi_vprintk);
printk(KERN_WARNING "NMI backtrace for cpu %d\n", cpu);
this_cpu_write(printk_func, printk_func_save);
cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
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