ring-buffer: do not grab locks in nmi

If ftrace_dump_on_oops is set, and an NMI detects a lockup, then it
will need to read from the ring buffer. But the read side of the
ring buffer still takes locks. This patch adds a check on the read
side that if it is in an NMI, then it will disable the ring buffer
and not take any locks.

Reads can still happen on a disabled ring buffer.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 51 insertions, 8 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 969f7cbe8e93..589b3eedfa67 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2466,6 +2466,21 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_iter_peek);
 
+static inline int rb_ok_to_lock(void)
+{
+        /*
+         * If an NMI die dumps out the content of the ring buffer
+         * do not grab locks. We also permanently disable the ring
+         * buffer too. A one time deal is all you get from reading
+         * the ring buffer from an NMI.
+         */
+        if (likely(!in_nmi() && !oops_in_progress))
+                return 1;
+
+        tracing_off_permanent();
+        return 0;
+}
+
 /**
  * ring_buffer_peek - peek at the next event to be read
  * @buffer: The ring buffer to read
@@ -2481,14 +2496,20 @@ ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
         struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
         struct ring_buffer_event *event;
         unsigned long flags;
+        int dolock;
 
         if (!cpumask_test_cpu(cpu, buffer->cpumask))
                 return NULL;
 
+        dolock = rb_ok_to_lock();
  again:
-        spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+        local_irq_save(flags);
+        if (dolock)
+                spin_lock(&cpu_buffer->reader_lock);
         event = rb_buffer_peek(buffer, cpu, ts);
-        spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+        if (dolock)
+                spin_unlock(&cpu_buffer->reader_lock);
+        local_irq_restore(flags);
 
         if (event && event->type_len == RINGBUF_TYPE_PADDING) {
                 cpu_relax();
@@ -2540,6 +2561,9 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
         struct ring_buffer_per_cpu *cpu_buffer;
         struct ring_buffer_event *event = NULL;
         unsigned long flags;
+        int dolock;
+
+        dolock = rb_ok_to_lock();
 
  again:
         /* might be called in atomic */
@@ -2549,7 +2573,9 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
                 goto out;
 
         cpu_buffer = buffer->buffers[cpu];
-        spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+        local_irq_save(flags);
+        if (dolock)
+                spin_lock(&cpu_buffer->reader_lock);
 
         event = rb_buffer_peek(buffer, cpu, ts);
         if (!event)
@@ -2558,7 +2584,9 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
         rb_advance_reader(cpu_buffer);
 
  out_unlock:
-        spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+        if (dolock)
+                spin_unlock(&cpu_buffer->reader_lock);
+        local_irq_restore(flags);
 
  out:
         preempt_enable();
@@ -2757,15 +2785,23 @@ int ring_buffer_empty(struct ring_buffer *buffer)
 {
         struct ring_buffer_per_cpu *cpu_buffer;
         unsigned long flags;
+        int dolock;
         int cpu;
         int ret;
 
+        dolock = rb_ok_to_lock();
+
         /* yes this is racy, but if you don't like the race, lock the buffer */
         for_each_buffer_cpu(buffer, cpu) {
                 cpu_buffer = buffer->buffers[cpu];
-                spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+                local_irq_save(flags);
+                if (dolock)
+                        spin_lock(&cpu_buffer->reader_lock);
                 ret = rb_per_cpu_empty(cpu_buffer);
-                spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+                if (dolock)
+                        spin_unlock(&cpu_buffer->reader_lock);
+                local_irq_restore(flags);
+
                 if (!ret)
                         return 0;
         }
@@ -2783,15 +2819,22 @@ int ring_buffer_empty_cpu(struct ring_buffer *buffer, int cpu)
 {
         struct ring_buffer_per_cpu *cpu_buffer;
         unsigned long flags;
+        int dolock;
         int ret;
 
         if (!cpumask_test_cpu(cpu, buffer->cpumask))
                 return 1;
 
+        dolock = rb_ok_to_lock();
+
         cpu_buffer = buffer->buffers[cpu];
-        spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+        local_irq_save(flags);
+        if (dolock)
+                spin_lock(&cpu_buffer->reader_lock);
         ret = rb_per_cpu_empty(cpu_buffer);
-        spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+        if (dolock)
+                spin_unlock(&cpu_buffer->reader_lock);
+        local_irq_restore(flags);
 
         return ret;
 }