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SubjectRe: [PATCH 3/6] tracing/ftrace: Fix a race condition in sched_switch tracer
2008/10/10 Steven Rostedt <rostedt@goodmis.org>:
>
> 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)

I didn't want to change it into an int because probe_sched_switch().
But actually that shouldn't be a problem.
Ok I will apply your comments in the next days (I can't until this week-end)....


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