linux-kernel - Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

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Date:   Fri, 16 Aug 2019 12:25:39 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

On Fri, 16 Aug 2019 10:26:43 -0400
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> Reading the sched_cmdline_ref and sched_tgid_ref initial state within
> tracing_start_sched_switch without holding the sched_register_mutex is
> racy against concurrent updates, which can lead to tracepoint probes
> being registered more than once (and thus trigger warnings within
> tracepoint.c).
> 
> Also, write and read to/from those variables should be done with
> WRITE_ONCE() and READ_ONCE(), given that those are read within tracing
> probes without holding the sched_register_mutex.
> 

I understand the READ_ONCE() but is the WRITE_ONCE() truly necessary?
It's done while holding the mutex. It's not that critical of a path,
and makes the code look ugly.

-- Steve



> [ Compile-tested only. I suspect it might fix the following syzbot
>   report:
> 
>   syzbot+774fddf07b7ab29a1e55@...kaller.appspotmail.com ]
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> CC: Joel Fernandes (Google) <joel@...lfernandes.org>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Steven Rostedt (VMware) <rostedt@...dmis.org>
> CC: Thomas Gleixner <tglx@...utronix.de>
> CC: Paul E. McKenney <paulmck@...ux.ibm.com>
> ---
>  kernel/trace/trace_sched_switch.c | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
> index e288168661e1..902e8bf59aeb 100644
> --- a/kernel/trace/trace_sched_switch.c
> +++ b/kernel/trace/trace_sched_switch.c
> @@ -26,8 +26,8 @@ probe_sched_switch(void *ignore, bool preempt,
>  {
>  	int flags;
>  
> -	flags = (RECORD_TGID * !!sched_tgid_ref) +
> -		(RECORD_CMDLINE * !!sched_cmdline_ref);
> +	flags = (RECORD_TGID * !!READ_ONCE(sched_tgid_ref)) +
> +		(RECORD_CMDLINE * !!READ_ONCE(sched_cmdline_ref));
>  
>  	if (!flags)
>  		return;
> @@ -39,8 +39,8 @@ probe_sched_wakeup(void *ignore, struct task_struct *wakee)
>  {
>  	int flags;
>  
> -	flags = (RECORD_TGID * !!sched_tgid_ref) +
> -		(RECORD_CMDLINE * !!sched_cmdline_ref);
> +	flags = (RECORD_TGID * !!READ_ONCE(sched_tgid_ref)) +
> +		(RECORD_CMDLINE * !!READ_ONCE(sched_cmdline_ref));
>  
>  	if (!flags)
>  		return;
> @@ -89,21 +89,28 @@ static void tracing_sched_unregister(void)
>  
>  static void tracing_start_sched_switch(int ops)
>  {
> -	bool sched_register = (!sched_cmdline_ref && !sched_tgid_ref);
> +	bool sched_register;
> +
>  	mutex_lock(&sched_register_mutex);
> +	sched_register = (!sched_cmdline_ref && !sched_tgid_ref);
>  
>  	switch (ops) {
>  	case RECORD_CMDLINE:
> -		sched_cmdline_ref++;
> +		WRITE_ONCE(sched_cmdline_ref, sched_cmdline_ref + 1);
>  		break;
>  
>  	case RECORD_TGID:
> -		sched_tgid_ref++;
> +		WRITE_ONCE(sched_tgid_ref, sched_tgid_ref + 1);
>  		break;
> +
> +	default:
> +		WARN_ONCE(1, "Unsupported tracing op: %d", ops);
> +		goto end;
>  	}
>  
> -	if (sched_register && (sched_cmdline_ref || sched_tgid_ref))
> +	if (sched_register)
>  		tracing_sched_register();
> +end:
>  	mutex_unlock(&sched_register_mutex);
>  }
>  
> @@ -113,16 +120,21 @@ static void tracing_stop_sched_switch(int ops)
>  
>  	switch (ops) {
>  	case RECORD_CMDLINE:
> -		sched_cmdline_ref--;
> +		WRITE_ONCE(sched_cmdline_ref, sched_cmdline_ref - 1);
>  		break;
>  
>  	case RECORD_TGID:
> -		sched_tgid_ref--;
> +		WRITE_ONCE(sched_tgid_ref, sched_tgid_ref - 1);
>  		break;
> +
> +	default:
> +		WARN_ONCE(1, "Unsupported tracing op: %d", ops);
> +		goto end;
>  	}
>  
>  	if (!sched_cmdline_ref && !sched_tgid_ref)
>  		tracing_sched_unregister();
> +end:
>  	mutex_unlock(&sched_register_mutex);
>  }
>