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Commit 8d707e8e authored by Steven Rostedt's avatar Steven Rostedt Committed by Steven Rostedt
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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: default avatarSteven Rostedt <rostedt@goodmis.org>
parent d4788207

kernel/trace/ring_buffer.c

+51 −8
Original line number Diff line number Diff line
@@ -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;

}