(depends on Tracing/fastboot: Enable boot tracing only during initcalls)
This patch fixes a race condition in the sched_switch tracer.
If several tasks (IE: concurrent initcalls) are playing with
tracing_start_cmdline_record() and tracing_stop_cmdline_record(),
the following situation could happen:

_ Task A and B are using the same tracepoint probe. Task A holds it. Task B is
sleeping and doesn't hold it.
_ Task A frees the sched tracer, then sched_ref is decremented to 0.
_ Task A is preempted and hadn't yet unregistered its tracepoint probe,
then B runs.
_ B increments sched_ref, sees it's 1 and then guess it has to register its probe.
But it has not been freed by task A.
_ A lot of bad things can happen after that...

Signed-off-by: Frederic Weisbecker
CC: Steven Rostedt
kernel/trace/trace_sched_switch.c | 5 +++++
1 files changed, 5 insertions(+), 0 deletions(-)
diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
index b8f56be..59de514 100644
--- a/kernel/trace/trace_sched_switch.c
+++ b/kernel/trace/trace_sched_switch.c
@@ -17,6 +17,7 @@
static struct trace_array *ctx_trace;
static int __read_mostly tracer_enabled;
static atomic_t sched_ref;
+static DEFINE_MUTEX(tracepoint_mutex);

static void
probe_sched_switch(struct rq *__rq, struct task_struct *prev,
@@ -125,18 +126,22 @@ static void tracing_start_sched_switch(void)
long ref;

+ mutex_lock(&tracepoint_mutex);
ref = atomic_inc_return(&sched_ref);
if (ref == 1)
+ mutex_unlock(&tracepoint_mutex);

static void tracing_stop_sched_switch(void)
long ref;

+ mutex_lock(&tracepoint_mutex);
ref = atomic_dec_and_test(&sched_ref);
if (ref)
+ mutex_unlock(&tracepoint_mutex);

void tracing_start_cmdline_record(void)
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