Messages in this thread | | | Date | Fri, 10 Oct 2008 09:39:17 -0400 (EDT) | From | Steven Rostedt <> | Subject | Re: [PATCH 3/6] tracing/ftrace: Fix a race condition in sched_switch tracer |
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On Fri, 10 Oct 2008, Frederic Weisbecker wrote: > > 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...
OK, I see the issue.
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > CC: Steven Rostedt <rostedt@goodmis.org> > --- > 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) > tracing_sched_register();
Could you make another patch to convert sched_ref to an int. With a mutex lock, there's no reason for this to be atomic.
We can simply do:
if (!(sched_ref++)) 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();
and
if (!(--sched_ref)) tracing_sched_unregister();
> + mutex_unlock(&tracepoint_mutex);
Also, lets change the name. sched_register_mutex?
> } > > void tracing_start_cmdline_record(void) >
-- Steve
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