Commit 34600f0e authored by Steven Rostedt's avatar Steven Rostedt Committed by Steven Rostedt
Browse files

tracing: Fix race with max_tr and changing tracers 



There's a race condition between the setting of a new tracer and
the update of the max trace buffers (the swap). When a new tracer
is added, it sets current_trace to nop_trace before disabling
the old tracer. At this moment, if the old tracer uses update_max_tr(),
the update may trigger the warning against !current_trace->use_max-tr,
as nop_trace doesn't have that set.

As update_max_tr() requires that interrupts be disabled, we can
add a check to see if current_trace == nop_trace and bail if it
does. Then when disabling the current_trace, set it to nop_trace
and run synchronize_sched(). This will make sure all calls to
update_max_tr() have completed (it was called with interrupts disabled).

As a clean up, this commit also removes shrinking and recreating
the max_tr buffer if the old and new tracers both have use_max_tr set.
The old way use to always shrink the buffer, and then expand it
for the next tracer. This is a waste of time.

Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
parent 0a71e4c6

kernel/trace/trace.c

+22 −7
Original line number Diff line number Diff line
@@ -709,10 +709,14 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu) 
		return;



	WARN_ON_ONCE(!irqs_disabled());

	if (!current_trace->use_max_tr) {

		WARN_ON_ONCE(1);



	/* If we disabled the tracer, stop now */

	if (current_trace == &nop_trace)

		return;

	}



	if (WARN_ON_ONCE(!current_trace->use_max_tr))

		return;



	arch_spin_lock(&ftrace_max_lock);



	tr->buffer = max_tr.buffer;
@@ -3185,6 +3189,7 @@ static int tracing_set_tracer(const char *buf) 
	static struct trace_option_dentry *topts;

	struct trace_array *tr = &global_trace;

	struct tracer *t;

	bool had_max_tr;

	int ret = 0;



	mutex_lock(&trace_types_lock);
@@ -3211,7 +3216,19 @@ static int tracing_set_tracer(const char *buf) 
	trace_branch_disable();

	if (current_trace && current_trace->reset)

		current_trace->reset(tr);

	if (current_trace && current_trace->use_max_tr) {



	had_max_tr = current_trace && current_trace->use_max_tr;

	current_trace = &nop_trace;



	if (had_max_tr && !t->use_max_tr) {

		/*

		 * We need to make sure that the update_max_tr sees that

		 * current_trace changed to nop_trace to keep it from

		 * swapping the buffers after we resize it.

		 * The update_max_tr is called from interrupts disabled

		 * so a synchronized_sched() is sufficient.

		 */

		synchronize_sched();

		/*

		 * We don't free the ring buffer. instead, resize it because

		 * The max_tr ring buffer has some state (e.g. ring->clock) and
@@ -3222,10 +3239,8 @@ static int tracing_set_tracer(const char *buf) 
	}

	destroy_trace_option_files(topts);



	current_trace = &nop_trace;



	topts = create_trace_option_files(t);

	if (t->use_max_tr) {

	if (t->use_max_tr && !had_max_tr) {

		/* we need to make per cpu buffer sizes equivalent */

		ret = resize_buffer_duplicate_size(&max_tr, &global_trace,

						   RING_BUFFER_ALL_CPUS);

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