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Commit 818e3dd3 authored by Steven Rostedt's avatar Steven Rostedt Committed by Ingo Molnar
Browse files

tracing, ring-buffer: add paranoid checks for loops 



While writing a new tracer, I had a bug where I caused the ring-buffer
to recurse in a bad way. The bug was with the tracer I was writing
and not the ring-buffer itself. But it took a long time to find the
problem.

This patch adds paranoid checks into the ring-buffer infrastructure
that will catch bugs of this nature.

Note: I put the bug back in the tracer and this patch showed the error
      nicely and prevented the lockup.

Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent b3aa5577

kernel/trace/ring_buffer.c

+56 −0
Original line number Diff line number Diff line
@@ -1022,8 +1022,23 @@ rb_reserve_next_event(struct ring_buffer_per_cpu *cpu_buffer, 
	struct ring_buffer_event *event;

	u64 ts, delta;

	int commit = 0;

	int nr_loops = 0;



 again:

	/*

	 * We allow for interrupts to reenter here and do a trace.

	 * If one does, it will cause this original code to loop

	 * back here. Even with heavy interrupts happening, this

	 * should only happen a few times in a row. If this happens

	 * 1000 times in a row, there must be either an interrupt

	 * storm or we have something buggy.

	 * Bail!

	 */

	if (unlikely(++nr_loops > 1000)) {

		RB_WARN_ON(cpu_buffer, 1);

		return NULL;

	}



	ts = ring_buffer_time_stamp(cpu_buffer->cpu);



	/*
@@ -1532,10 +1547,23 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) 
{

	struct buffer_page *reader = NULL;

	unsigned long flags;

	int nr_loops = 0;



	spin_lock_irqsave(&cpu_buffer->lock, flags);



 again:

	/*

	 * This should normally only loop twice. But because the

	 * start of the reader inserts an empty page, it causes

	 * a case where we will loop three times. There should be no

	 * reason to loop four times (that I know of).

	 */

	if (unlikely(++nr_loops > 3)) {

		RB_WARN_ON(cpu_buffer, 1);

		reader = NULL;

		goto out;

	}



	reader = cpu_buffer->reader_page;



	/* If there's more to read, return this page */
@@ -1665,6 +1693,7 @@ ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts) 
	struct ring_buffer_per_cpu *cpu_buffer;

	struct ring_buffer_event *event;

	struct buffer_page *reader;

	int nr_loops = 0;



	if (!cpu_isset(cpu, buffer->cpumask))

		return NULL;
@@ -1672,6 +1701,19 @@ ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts) 
	cpu_buffer = buffer->buffers[cpu];



 again:

	/*

	 * We repeat when a timestamp is encountered. It is possible

	 * to get multiple timestamps from an interrupt entering just

	 * as one timestamp is about to be written. The max times

	 * that this can happen is the number of nested interrupts we

	 * can have.  Nesting 10 deep of interrupts is clearly

	 * an anomaly.

	 */

	if (unlikely(++nr_loops > 10)) {

		RB_WARN_ON(cpu_buffer, 1);

		return NULL;

	}



	reader = rb_get_reader_page(cpu_buffer);

	if (!reader)

		return NULL;
@@ -1722,6 +1764,7 @@ ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts) 
	struct ring_buffer *buffer;

	struct ring_buffer_per_cpu *cpu_buffer;

	struct ring_buffer_event *event;

	int nr_loops = 0;



	if (ring_buffer_iter_empty(iter))

		return NULL;
@@ -1730,6 +1773,19 @@ ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts) 
	buffer = cpu_buffer->buffer;



 again:

	/*

	 * We repeat when a timestamp is encountered. It is possible

	 * to get multiple timestamps from an interrupt entering just

	 * as one timestamp is about to be written. The max times

	 * that this can happen is the number of nested interrupts we

	 * can have. Nesting 10 deep of interrupts is clearly

	 * an anomaly.

	 */

	if (unlikely(++nr_loops > 10)) {

		RB_WARN_ON(cpu_buffer, 1);

		return NULL;

	}



	if (rb_per_cpu_empty(cpu_buffer))

		return NULL;