Messages in this thread | | | Date | Fri, 10 Oct 2008 15:51:13 +0200 | From | "Frédéric Weisbecker" <> | Subject | Re: [PATCH 3/6] tracing/ftrace: Fix a race condition in sched_switch tracer |
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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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