Re: [PATCH v5 1/6] kprobes: Remove dependency to the module_mutex

[Jarkko Sakkinen]

On Tue, Jul 28, 2020 at 04:34:00PM +0900, Masami Hiramatsu wrote:
> On Sat, 25 Jul 2020 12:21:10 +0200
> Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> 
> > 
> > * Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
> > 
> > > On Fri, Jul 24, 2020 at 11:17:11AM +0200, Ingo Molnar wrote:
> > > > 
> > > > * Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> 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.
> > 
> > If it's unnecessary, then it needs to be removed.
> > 
> > If it's necessary, then it needs to be documented better.
> 
> Good catch! This is not needed anymore.
> It has been introduced to avoid conflict with text modification, at that
> point we didn't get text_mutex. But after commit f1c6ece23729 ("kprobes: Fix 
> potential deadlock in kprobe_optimizer()") moved the text_mutex in the current
> position, we don't need it. (and anyway, keeping kprobe_mutex locked means
> any module unloading will be stopped inside kprobes_module_callback())
> 
> This may help?

Hey, thanks a lot. This will help to clean my patch set. I'll send a
follow up version as soon I'm on track with my work.  I have to recall
my set of changes and backtrack some of the discussion.

I was two weeks in vacation and last week had bunch of network
connectivity issues last week. Anyway, enough time for details to fade
away :-)

/Jarkko

> 
> From 2355ecd941c3234b12a6de7443592848ed4e2087 Mon Sep 17 00:00:00 2001
> From: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Date: Tue, 28 Jul 2020 16:32:34 +0900
> Subject: [PATCH] kprobes: Remove unneeded module_mutex lock from the optimizer
> 
> Remove unneeded module_mutex locking from the optimizer. Since
> we already locks both kprobe_mutex and text_mutex in the optimizer,
> text will not be changed and the module unloading will be stopped
> inside kprobes_module_callback().
> 
> Suggested-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> ---
>  kernel/kprobes.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 4a904cc56d68..d1b02e890793 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -563,8 +563,6 @@ static void kprobe_optimizer(struct work_struct *work)
>  	mutex_lock(&kprobe_mutex);
>  	cpus_read_lock();
>  	mutex_lock(&text_mutex);
> -	/* Lock modules while optimizing kprobes */
> -	mutex_lock(&module_mutex);
>  
>  	/*
>  	 * Step 1: Unoptimize kprobes and collect cleaned (unused and disarmed)
> @@ -589,7 +587,6 @@ static void kprobe_optimizer(struct work_struct *work)
>  	/* Step 4: Free cleaned kprobes after quiesence period */
>  	do_free_cleaned_kprobes();
>  
> -	mutex_unlock(&module_mutex);
>  	mutex_unlock(&text_mutex);
>  	cpus_read_unlock();
>  
> -- 
> 2.25.1
> -- 
> Masami Hiramatsu <mhiramat@xxxxxxxxxx>