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>

1 files changed, 86 insertions, 5 deletions

diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index 6a1e71bde323..c95c3e9ce196 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/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;
         }