lib/nmi_backtrace.c

   1 /*
   2  *  NMI backtrace support
   3  *
   4  * Gratuitously copied from arch/x86/kernel/apic/hw_nmi.c by Russell King,
   5  * with the following header:
   6  *
   7  *  HW NMI watchdog support
   8  *
   9  *  started by Don Zickus, Copyright (C) 2010 Red Hat, Inc.
  10  *
  11  *  Arch specific calls to support NMI watchdog
  12  *
  13  *  Bits copied from original nmi.c file
  14  */
  15 #include <linux/cpumask.h>
  16 #include <linux/delay.h>
  17 #include <linux/kprobes.h>
  18 #include <linux/nmi.h>
  19 #include <linux/seq_buf.h>
  20
  21 #ifdef arch_trigger_all_cpu_backtrace
  22 /* For reliability, we're prepared to waste bits here. */
  23 static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
  24 static cpumask_t printtrace_mask;
  25
  26 #define NMI_BUF_SIZE            4096
  27
  28 struct nmi_seq_buf {
  29         unsigned char           buffer[NMI_BUF_SIZE];
  30         struct seq_buf          seq;
  31 };
  32
  33 /* Safe printing in NMI context */
  34 static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
  35
  36 /* "in progress" flag of arch_trigger_all_cpu_backtrace */
  37 static unsigned long backtrace_flag;
  38
  39 static void print_seq_line(struct nmi_seq_buf *s, int start, int end)
  40 {
  41         const char *buf = s->buffer + start;
  42
  43         printk("%.*s", (end - start) + 1, buf);
  44 }
  45
  46 void nmi_trigger_all_cpu_backtrace(bool include_self,
  47                                    void (*raise)(cpumask_t *mask))
  48 {
  49         struct nmi_seq_buf *s;
  50         int i, cpu, this_cpu = get_cpu();
  51
  52         if (test_and_set_bit(0, &backtrace_flag)) {
  53                 /*
  54                  * If there is already a trigger_all_cpu_backtrace() in progress
  55                  * (backtrace_flag == 1), don't output double cpu dump infos.
  56                  */
  57                 put_cpu();
  58                 return;
  59         }
  60
  61         cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
  62         if (!include_self)
  63                 cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_mask));
  64
  65         cpumask_copy(&printtrace_mask, to_cpumask(backtrace_mask));
  66
  67         /*
  68          * Set up per_cpu seq_buf buffers that the NMIs running on the other
  69          * CPUs will write to.
  70          */
  71         for_each_cpu(cpu, to_cpumask(backtrace_mask)) {
  72                 s = &per_cpu(nmi_print_seq, cpu);
  73                 seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
  74         }
  75
  76         if (!cpumask_empty(to_cpumask(backtrace_mask))) {
  77                 pr_info("Sending NMI to %s CPUs:\n",
  78                         (include_self ? "all" : "other"));
  79                 raise(to_cpumask(backtrace_mask));
  80         }
  81
  82         /* Wait for up to 10 seconds for all CPUs to do the backtrace */
  83         for (i = 0; i < 10 * 1000; i++) {
  84                 if (cpumask_empty(to_cpumask(backtrace_mask)))
  85                         break;
  86                 mdelay(1);
  87                 touch_softlockup_watchdog();
  88         }
  89
  90         /*
  91          * Now that all the NMIs have triggered, we can dump out their
  92          * back traces safely to the console.
  93          */
  94         for_each_cpu(cpu, &printtrace_mask) {
  95                 int len, last_i = 0;
  96
  97                 s = &per_cpu(nmi_print_seq, cpu);
  98                 len = seq_buf_used(&s->seq);
  99                 if (!len)
 100                         continue;
 101
 102                 /* Print line by line. */
 103                 for (i = 0; i < len; i++) {
 104                         if (s->buffer[i] == '\n') {
 105                                 print_seq_line(s, last_i, i);
 106                                 last_i = i + 1;
 107                         }
 108                 }
 109                 /* Check if there was a partial line. */
 110                 if (last_i < len) {
 111                         print_seq_line(s, last_i, len - 1);
 112                         pr_cont("\n");
 113                 }
 114         }
 115
 116         clear_bit(0, &backtrace_flag);
 117         smp_mb__after_atomic();
 118         put_cpu();
 119 }
 120
 121 /*
 122  * It is not safe to call printk() directly from NMI handlers.
 123  * It may be fine if the NMI detected a lock up and we have no choice
 124  * but to do so, but doing a NMI on all other CPUs to get a back trace
 125  * can be done with a sysrq-l. We don't want that to lock up, which
 126  * can happen if the NMI interrupts a printk in progress.
 127  *
 128  * Instead, we redirect the vprintk() to this nmi_vprintk() that writes
 129  * the content into a per cpu seq_buf buffer. Then when the NMIs are
 130  * all done, we can safely dump the contents of the seq_buf to a printk()
 131  * from a non NMI context.
 132  */
 133 static int nmi_vprintk(const char *fmt, va_list args)
 134 {
 135         struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
 136         unsigned int len = seq_buf_used(&s->seq);
 137
 138         seq_buf_vprintf(&s->seq, fmt, args);
 139         return seq_buf_used(&s->seq) - len;
 140 }
 141
 142 bool nmi_cpu_backtrace(struct pt_regs *regs)
 143 {
 144         int cpu = smp_processor_id();
 145
 146         if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
 147                 printk_func_t printk_func_save = this_cpu_read(printk_func);
 148
 149                 /* Replace printk to write into the NMI seq */
 150                 this_cpu_write(printk_func, nmi_vprintk);
 151                 pr_warn("NMI backtrace for cpu %d\n", cpu);
 152                 show_regs(regs);
 153                 this_cpu_write(printk_func, printk_func_save);
 154
 155                 cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
 156                 return true;
 157         }
 158
 159         return false;
 160 }
 161 NOKPROBE_SYMBOL(nmi_cpu_backtrace);
 162 #endif