linux-kernel - Re: [PATCH 04/11] ftrace: comment arch ftrace code

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Date:	Wed, 22 Oct 2008 15:16:33 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, fweisbec@...il.com,
	sagar.abhishek@...il.com, davem@...emloft.net, tglx@...utronix.de,
	peterz@...radead.org, torvalds@...ux-foundation.org,
	srostedt@...hat.com
Subject: Re: [PATCH 04/11] ftrace: comment arch ftrace code


On Wed, 22 Oct 2008, Andrew Morton wrote:
> 
> I dunno.
> 
> __copy_to_user_inatomic() is for "copying memory from userspace while
> in an atomic context".
> 
> But what you're doing here is "modifying some kernel text which might
> generate a fault".  It seems somewhat interface-abusive to use a
> userspace access function for that just because it happens right now to
> do the right thing.
> 
> I'd suggest that for clarity and for future-safety, you create some new
> interface function which does that thing.  Right now it can be a simple
> wrapper around __copy_from_user_inatomic().
> 
> <looks>
> 
> oh, someone added one - probe_kernel_write().  Why not use that?

I didn't know about that code. That is what I want.

-- Steve

> 
> <wonders why he doesn't know what's going on any more>
> 
> 
> Also, I hope that the above code is called from within a
> pagefault_disable()d region?  Or are relying upon some magical
> side-effect of something which happens to do the same thing as
> pagefault_disable()?  IOW: by what means does the above code ensure
> that do_page_fault() will see in_atomic()==true?

This code is called from kstop_machine, or simply has interrupts disabled.

-- Steve

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