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>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 969f7cbe8e9375cf6bffc20d2b5afcac04be80eb..589b3eedfa67a2f1b25b70684920d6c59018c646 100644 (file)
--- 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;
 }