linux-kernel - Re: [PATCH] Add a text

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Message-ID: <alpine.LRH.2.00.1311211059260.9932@twin.jikos.cz>
Date:	Thu, 21 Nov 2013 11:02:14 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andy Lutomirski <luto@...capital.net>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] Add a text_poke syscall

On Tue, 19 Nov 2013, Andi Kleen wrote:

> > > +	err = get_user_pages_fast((unsigned long)addr, npages, 1, pages);
> > > +	if (err < 0)
> > > +		return err;
> > > +	err = 0;
> > > +	mutex_lock(&text_mutex);
> > > +	bp_target_mm = current->mm;
> > > +	bp_int3_addr = (u8 *)addr + 1;
> > 
> > Do you need an smp_wmb here?  (Maybe there's a strong enough barrier in
> > __text_poke_bp.)
> 
> __text_poke_bp already has enough barriers (although I don't
> think they are really needed in any case)

As an author of those barriers I have to ask -- why do you think so? The 
first one is there to avoid int3 handler seeing !patching_in_progress 
while it has actually been entered due to text_poke_bp() kicking in.

The second one is there for exactly the inverse reason.

Both are pairing to smp_rmb() in poke_int3_handler().

Thanks,

-- 
Jiri Kosina
SUSE Labs

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