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>

diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index 6a1e71bde323360d976e58b49ca231443e3e0584..c95c3e9ce196ead03da7ae07fd741f3421df6df1 100644 (file)
--- 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/seq_buf.h>
 
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 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)
                touch_softlockup_watchdog();
        }
 
+       /*
+        * 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)
+                       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;
+                       }
+               }
+               /* Check if there was a partial line. */
+               if (last_i < len) {
+                       print_seq_line(s, last_i, len - 1);
+                       pr_cont("\n");
+               }
+       }
+
        clear_bit(0, &backtrace_flag);
        smp_mb__after_atomic();
        put_cpu();
 }
 
+/*
+ * 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);
 
-               arch_spin_lock(&lock);
+               /* 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);
                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;
        }