Re: [PATCH v2 2/2] tracing: protect trace_probe_log with mutex

[Paul Cacheux <paulcacheux@gmail.com>]

On Fri, 2025-05-02 at 09:50 -0400, Steven Rostedt wrote:
> On Fri, 02 May 2025 15:15:53 +0200
> Paul Cacheux via B4 Relay <devnull+paulcacheux.gmail.com@kernel.org>
> wrote:
> 
> > From: Paul Cacheux <paulcacheux@gmail.com>
> > 
> > The shared trace_probe_log variable can be accessed and modified
> > by multiple processes using tracefs at the same time, this new
> > mutex will guarantee it's always in a coherent state.
> > 
> > There is no guarantee that multiple errors happening at the same
> > time will each have the correct error message, but at least this
> > won't crash.
> > 
> > Fixes: ab105a4fb894 ("tracing: Use tracing error_log with probe
> > events")
> > 
> 
> No space needed between Fixes and SOB.

Sorry about that, fixed in v3

> 
> > Signed-off-by: Paul Cacheux <paulcacheux@gmail.com>
> > ---
> >  kernel/trace/trace_probe.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/kernel/trace/trace_probe.c
> > b/kernel/trace/trace_probe.c
> > index
> > 2eeecb6c95eea55502b83af6775b7b6f0cc5ab94..14a7a0b59cd20a8bc43e3e7c6
> > 53e986081f924c8 100644
> > --- a/kernel/trace/trace_probe.c
> > +++ b/kernel/trace/trace_probe.c
> > @@ -154,9 +154,11 @@ static const struct fetch_type
> > *find_fetch_type(const char *type, unsigned long
> >  }
> >  
> >  static struct trace_probe_log trace_probe_log;
> > +static DEFINE_MUTEX(trace_probe_log_lock);
> 
> Probably should add a comment here saying something like:
> 
> /*
>  * The trace_probe_log_lock only protects against the individual
>  * modification of the trace_probe_log. It does not protect against
>  * the log from producing garbage if two probes use it at the same
>  * time. That would only happen if two admins were trying to add
>  * probes simultaneously which they shouldn't be doing.
>  */

I sent a v3 with the space removed and this comment added, thank you
for the review.

> 
> -- Steve
> 
> 
> >  
> >  void trace_probe_log_init(const char *subsystem, int argc, const
> > char **argv)
> >  {
> > +	guard(mutex)(&trace_probe_log_lock);
> >  	trace_probe_log.subsystem = subsystem;
> >  	trace_probe_log.argc = argc;
> >  	trace_probe_log.argv = argv;
> > @@ -165,11 +167,13 @@ void trace_probe_log_init(const char
> > *subsystem, int argc, const char **argv)
> >  
> >  void trace_probe_log_clear(void)
> >  {
> > +	guard(mutex)(&trace_probe_log_lock);
> >  	memset(&trace_probe_log, 0, sizeof(trace_probe_log));
> >  }
> >  
> >  void trace_probe_log_set_index(int index)
> >  {
> > +	guard(mutex)(&trace_probe_log_lock);
> >  	trace_probe_log.index = index;
> >  }
> >  
> > @@ -178,6 +182,8 @@ void __trace_probe_log_err(int offset, int
> > err_type)
> >  	char *command, *p;
> >  	int i, len = 0, pos = 0;
> >  
> > +	guard(mutex)(&trace_probe_log_lock);
> > +
> >  	if (!trace_probe_log.argv)
> >  		return;
> >  
> > 

Best regards,

Paul Cacheux