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Date:   Sat, 25 Jul 2020 06:01:07 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andi Kleen <ak@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jessica Yu <jeyu@kernel.org>,
        "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH v5 1/6] kprobes: Remove dependency to the module_mutex
Message-ID: <20200725030107.GF17052@linux.intel.com>
References: <20200724050553.1724168-1-jarkko.sakkinen@linux.intel.com>
 <20200724050553.1724168-2-jarkko.sakkinen@linux.intel.com>
 <20200724091711.GB517988@gmail.com>
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On Fri, Jul 24, 2020 at 11:17:11AM +0200, Ingo Molnar wrote:
> 
> * Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> wrote:
> 
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -564,7 +564,7 @@ static void kprobe_optimizer(struct work_struct *work)
> >  	cpus_read_lock();
> >  	mutex_lock(&text_mutex);
> >  	/* Lock modules while optimizing kprobes */
> > -	mutex_lock(&module_mutex);
> > +	lock_modules();
> >  
> >  	/*
> >  	 * Step 1: Unoptimize kprobes and collect cleaned (unused and disarmed)
> > @@ -589,7 +589,7 @@ static void kprobe_optimizer(struct work_struct *work)
> >  	/* Step 4: Free cleaned kprobes after quiesence period */
> >  	do_free_cleaned_kprobes();
> >  
> > -	mutex_unlock(&module_mutex);
> > +	unlock_modules();
> >  	mutex_unlock(&text_mutex);
> >  	cpus_read_unlock();
> 
> BTW., it would be nice to expand on the comments above - exactly which 
> parts of the modules code is being serialized against and why?
> 
> We already hold the text_mutex here, which should protect against most 
> kprobes related activities interfering - and it's unclear (to me) 
> which part of the modules code is being serialized with here, and the 
> 'lock modules while optimizing kprobes' comments is unhelpful. :-)
> 
> Thanks,
> 
> 	Ingo

AFAIK, only if you need to call find_module(), you ever need to acquire
this mutex. 99% of time it is internally taken care by kernel/module.c.

I cannot make up any obvious reason to acquire it here.

/Jarkko