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. Link: http://lkml.kernel.org/p/20140619213952.360076309@goodmis.org Link: http://lkml.kernel.org/r/20141115050605.055232587@goodmis.org Tested-by: Jiri Kosina <jkosina@suse.cz> Acked-by: Jiri Kosina <jkosina@suse.cz> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Petr Mladek <pmladek@suse.cz> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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@ -18,6 +18,7 @@
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#include <linux/nmi.h>
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#include <linux/module.h>
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#include <linux/delay.h>
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#include <linux/seq_buf.h>
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#ifdef CONFIG_HARDLOCKUP_DETECTOR
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u64 hw_nmi_get_sample_period(int watchdog_thresh)
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@ -29,14 +30,35 @@ u64 hw_nmi_get_sample_period(int watchdog_thresh)
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#ifdef arch_trigger_all_cpu_backtrace
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/* For reliability, we're prepared to waste bits here. */
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static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
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static cpumask_var_t printtrace_mask;
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#define NMI_BUF_SIZE 4096
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struct nmi_seq_buf {
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unsigned char buffer[NMI_BUF_SIZE];
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struct seq_buf seq;
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};
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/* Safe printing in NMI context */
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static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
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/* "in progress" flag of arch_trigger_all_cpu_backtrace */
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static unsigned long backtrace_flag;
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static void print_seq_line(struct nmi_seq_buf *s, int start, int end)
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{
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const char *buf = s->buffer + start;
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printk("%.*s", (end - start) + 1, buf);
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}
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void arch_trigger_all_cpu_backtrace(bool include_self)
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{
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struct nmi_seq_buf *s;
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int len;
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int cpu;
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int i;
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int cpu = get_cpu();
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int this_cpu = get_cpu();
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if (test_and_set_bit(0, &backtrace_flag)) {
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/*
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@ -49,7 +71,17 @@ void arch_trigger_all_cpu_backtrace(bool include_self)
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cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
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if (!include_self)
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cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
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cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_mask));
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cpumask_copy(printtrace_mask, to_cpumask(backtrace_mask));
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/*
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* Set up per_cpu seq_buf buffers that the NMIs running on the other
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* CPUs will write to.
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*/
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for_each_cpu(cpu, to_cpumask(backtrace_mask)) {
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s = &per_cpu(nmi_print_seq, cpu);
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seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
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}
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if (!cpumask_empty(to_cpumask(backtrace_mask))) {
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pr_info("sending NMI to %s CPUs:\n",
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@ -65,11 +97,58 @@ void arch_trigger_all_cpu_backtrace(bool include_self)
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touch_softlockup_watchdog();
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}
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/*
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* Now that all the NMIs have triggered, we can dump out their
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* back traces safely to the console.
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*/
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for_each_cpu(cpu, printtrace_mask) {
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int last_i = 0;
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s = &per_cpu(nmi_print_seq, cpu);
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len = seq_buf_used(&s->seq);
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if (!len)
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continue;
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/* Print line by line. */
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for (i = 0; i < len; i++) {
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if (s->buffer[i] == '\n') {
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print_seq_line(s, last_i, i);
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last_i = i + 1;
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}
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}
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/* Check if there was a partial line. */
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if (last_i < len) {
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print_seq_line(s, last_i, len - 1);
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pr_cont("\n");
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}
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}
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clear_bit(0, &backtrace_flag);
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smp_mb__after_atomic();
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put_cpu();
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}
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/*
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* It is not safe to call printk() directly from NMI handlers.
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* It may be fine if the NMI detected a lock up and we have no choice
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* but to do so, but doing a NMI on all other CPUs to get a back trace
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* can be done with a sysrq-l. We don't want that to lock up, which
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* can happen if the NMI interrupts a printk in progress.
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*
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* Instead, we redirect the vprintk() to this nmi_vprintk() that writes
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* the content into a per cpu seq_buf buffer. Then when the NMIs are
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* all done, we can safely dump the contents of the seq_buf to a printk()
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* from a non NMI context.
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*/
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static int nmi_vprintk(const char *fmt, va_list args)
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{
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struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
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unsigned int len = seq_buf_used(&s->seq);
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seq_buf_vprintf(&s->seq, fmt, args);
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return seq_buf_used(&s->seq) - len;
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}
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static int
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arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
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{
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@ -78,12 +157,14 @@ arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
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cpu = smp_processor_id();
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if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
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static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
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printk_func_t printk_func_save = this_cpu_read(printk_func);
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arch_spin_lock(&lock);
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/* Replace printk to write into the NMI seq */
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this_cpu_write(printk_func, nmi_vprintk);
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printk(KERN_WARNING "NMI backtrace for cpu %d\n", cpu);
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show_regs(regs);
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arch_spin_unlock(&lock);
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this_cpu_write(printk_func, printk_func_save);
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cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
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return NMI_HANDLED;
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}
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