linux-kernel - Re: [PATCH 1/2] libtracefs: Add random number to keep synthetic variables unique

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Date:   Thu, 12 Aug 2021 10:00:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yordan Karadzhov <y.karadz@...il.com>
Cc:     linux-trace-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tom Zanussi <zanussi@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH 1/2] libtracefs: Add random number to keep synthetic
 variables unique

On Thu, 12 Aug 2021 11:34:57 +0300
Yordan Karadzhov <y.karadz@...il.com> wrote:

> > @@ -957,7 +960,15 @@ static char *new_arg(struct tracefs_synth *synth)
> >   	char *arg;
> >   	int ret;
> >   
> > -	ret = asprintf(&arg, "__arg__%d", cnt);
> > +	/* Create a unique argument name */
> > +	if (!synth->arg_name[0]) {
> > +		srand(time(NULL));  
> 
> Nit: Have in mind that time(NULL) has 1 second resolution. Fast consecutive calls (within a second) of this function can 
> generate identical random numbers.
> This can be mitigated if we do something like this:
> 
> 		struct timeval now;
> 
> 		gettimeofday(&now, NULL);
> 		srand(now.tv_usec);

So you are saying that if one thread created two synthetic events
within a second, then this could give the same value. Yeah, I can see
that could happen. I was hoping to avoid the declaring the "now" and
calling gettimeofday().

Also, looking more into this, I see that rand() is not safe in thread
context (it may not be a problem, but there's no guarantee), and
perhaps we should just open code it, to be on the safe side.

Thanks for the review.

-- Steve