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From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: linux-kernel@vger.kernel.org,
	Joel Fernandes <joel@joelfernandes.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E . McKenney" <paulmck@linux.ibm.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
Date: Fri, 16 Aug 2019 12:25:39 -0400	[thread overview]
Message-ID: <20190816122539.34fada7b@oasis.local.home> (raw)
In-Reply-To: <20190816142643.13758-1-mathieu.desnoyers@efficios.com>

On Fri, 16 Aug 2019 10:26:43 -0400
Mathieu Desnoyers <mathieu.desnoyers@efficios.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@syzkaller.appspotmail.com ]
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: Joel Fernandes (Google) <joel@joelfernandes.org>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Steven Rostedt (VMware) <rostedt@goodmis.org>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Paul E. McKenney <paulmck@linux.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);
>  }
>  


  reply	other threads:[~2019-08-16 16:25 UTC|newest]

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-18 10:29 WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16  0:11 ` syzbot
2019-08-16 14:26   ` [PATCH 1/1] Fix: trace sched switch start/stop racy updates Mathieu Desnoyers
2019-08-16 16:25     ` Steven Rostedt [this message]
2019-08-16 16:48       ` Valentin Schneider
2019-08-16 17:04         ` Steven Rostedt
2019-08-16 17:41           ` Mathieu Desnoyers
2019-08-16 19:18             ` Steven Rostedt
2019-08-16 19:19             ` Alan Stern
2019-08-16 20:44               ` Joel Fernandes
2019-08-16 20:49                 ` Thomas Gleixner
2019-08-16 20:57                   ` Joel Fernandes
2019-08-16 22:27                     ` Valentin Schneider
2019-08-16 22:57                       ` Linus Torvalds
2019-08-17  1:41                         ` Mathieu Desnoyers
2019-08-17  4:52                         ` Paul E. McKenney
2019-08-17  8:28                           ` Linus Torvalds
2019-08-17  8:44                             ` Linus Torvalds
2019-08-17 15:02                               ` Mathieu Desnoyers
2019-08-17 20:03                                 ` Valentin Schneider
2019-08-17 23:00                                   ` Paul E. McKenney
2019-08-19 10:34                                     ` Valentin Schneider
2019-08-17 22:28                             ` Paul E. McKenney
2019-08-20 14:01                           ` Peter Zijlstra
2019-08-20 20:31                             ` Paul E. McKenney
2019-08-20 20:39                               ` Peter Zijlstra
2019-08-20 20:52                                 ` Paul E. McKenney
2019-08-16 21:04                   ` Linus Torvalds
2019-08-17  1:36                     ` Mathieu Desnoyers
2019-08-17  2:13                       ` Steven Rostedt
2019-08-17 14:40                         ` Mathieu Desnoyers
2019-08-17 15:26                           ` Steven Rostedt
2019-08-17 15:55                             ` Mathieu Desnoyers
2019-08-17 16:40                               ` Steven Rostedt
2019-08-17 22:06                                 ` Paul E. McKenney
2019-08-17  8:08                       ` Linus Torvalds
2019-08-20 13:56                         ` Peter Zijlstra
2019-08-20 20:29                           ` Paul E. McKenney
2019-08-21 10:32                             ` Will Deacon
2019-08-21 13:23                               ` Paul E. McKenney
2019-08-21 13:32                                 ` Will Deacon
2019-08-21 13:56                                   ` Paul E. McKenney
2019-08-21 16:22                                     ` Will Deacon
2019-08-21 15:33                                 ` Peter Zijlstra
2019-08-21 15:48                                   ` Mathieu Desnoyers
2019-08-21 16:14                                     ` Paul E. McKenney
2019-08-21 19:03                                     ` Joel Fernandes
2019-09-09  6:21                           ` Herbert Xu
2019-08-16 20:49                 ` Steven Rostedt
2019-08-16 20:59                   ` Joel Fernandes
2019-08-17  1:25                   ` Mathieu Desnoyers
2019-08-18  9:15                   ` stable markup was " Pavel Machek
2019-08-16 17:19       ` Mathieu Desnoyers
2019-08-16 19:15         ` Steven Rostedt
2019-08-17 14:27           ` Mathieu Desnoyers
2019-08-17 15:42             ` Steven Rostedt
2019-08-17 15:53               ` Mathieu Desnoyers
2019-08-17 16:43                 ` Steven Rostedt
2019-08-16 12:32 ` WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16 12:41   ` Sebastian Andrzej Siewior

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