2 files changed, 54 insertions, 2 deletions

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 28f2644484d9..25131a5d5e4f 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -9,6 +9,9 @@ config USER_STACKTRACE_SUPPORT
 config NOP_TRACER
         bool
 
+config HAVE_FTRACE_NMI_ENTER
+        bool
+
 config HAVE_FUNCTION_TRACER
         bool
 
@@ -37,6 +40,11 @@ config TRACER_MAX_TRACE
 config RING_BUFFER
         bool
 
+config FTRACE_NMI_ENTER
+       bool
+       depends on HAVE_FTRACE_NMI_ENTER
+       default y
+
 config TRACING
         bool
         select DEBUG_FS
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b36d7374ceef..a60a6a852f42 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4,6 +4,7 @@
  * Copyright (C) 2008 Steven Rostedt <srostedt@redhat.com>
  */
 #include <linux/ring_buffer.h>
+#include <linux/ftrace_irq.h>
 #include <linux/spinlock.h>
 #include <linux/debugfs.h>
 #include <linux/uaccess.h>
@@ -19,6 +20,35 @@
 #include "trace.h"
 
 /*
+ * Since the write to the buffer is still not fully lockless,
+ * we must be careful with NMIs. The locks in the writers
+ * are taken when a write crosses to a new page. The locks
+ * protect against races with the readers (this will soon
+ * be fixed with a lockless solution).
+ *
+ * Because we can not protect against NMIs, and we want to
+ * keep traces reentrant, we need to manage what happens
+ * when we are in an NMI.
+ */
+static DEFINE_PER_CPU(int, rb_in_nmi);
+
+void ftrace_nmi_enter(void)
+{
+        __get_cpu_var(rb_in_nmi)++;
+        /* call arch specific handler too */
+        arch_ftrace_nmi_enter();
+}
+
+void ftrace_nmi_exit(void)
+{
+        arch_ftrace_nmi_exit();
+        __get_cpu_var(rb_in_nmi)--;
+        /* NMIs are not recursive */
+        WARN_ON_ONCE(__get_cpu_var(rb_in_nmi));
+}
+
+
+/*
  * A fast way to enable or disable all ring buffers is to
  * call tracing_on or tracing_off. Turning off the ring buffers
  * prevents all ring buffers from being recorded to.
@@ -982,6 +1012,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
         struct ring_buffer *buffer = cpu_buffer->buffer;
         struct ring_buffer_event *event;
         unsigned long flags;
+        bool lock_taken = false;
 
         commit_page = cpu_buffer->commit_page;
         /* we just need to protect against interrupts */
@@ -995,7 +1026,19 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
                 struct buffer_page *next_page = tail_page;
 
                 local_irq_save(flags);
-                __raw_spin_lock(&cpu_buffer->lock);
+                /*
+                 * NMIs can happen after we take the lock.
+                 * If we are in an NMI, only take the lock
+                 * if it is not already taken. Otherwise
+                 * simply fail.
+                 */
+                if (unlikely(__get_cpu_var(rb_in_nmi))) {
+                        if (!__raw_spin_trylock(&cpu_buffer->lock))
+                                goto out_unlock;
+                } else
+                        __raw_spin_lock(&cpu_buffer->lock);
+
+                lock_taken = true;
 
                 rb_inc_page(cpu_buffer, &next_page);
 
@@ -1097,7 +1140,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
         if (tail <= BUF_PAGE_SIZE)
                 local_set(&tail_page->write, tail);
 
-        __raw_spin_unlock(&cpu_buffer->lock);
+        if (likely(lock_taken))
+                __raw_spin_unlock(&cpu_buffer->lock);
         local_irq_restore(flags);
         return NULL;
 }