tracing/ftrace: fix a race condition in sched_switch tracer
Impact: fix race condition in sched_switch tracer
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 <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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 @@
{
long ref;
+ mutex_lock(&tracepoint_mutex);
ref = atomic_inc_return(&sched_ref);
if (ref == 1)
tracing_sched_register();
+ 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)
tracing_sched_unregister();
+ mutex_unlock(&tracepoint_mutex);
}
void tracing_start_cmdline_record(void)