linux-kernel - Re: A bug in ftrace

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Message-ID: <20160811100146.4cb94980@gandalf.local.home>
Date:	Thu, 11 Aug 2016 10:01:46 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: A bug in ftrace - dynamic fops

On Thu, 11 Aug 2016 10:46:53 +0200 (CEST)
Miroslav Benes <mbenes@...e.cz> wrote:

> On Tue, 9 Aug 2016, Steven Rostedt wrote:
> 
> > On Tue, 9 Aug 2016 10:16:00 +0200 (CEST)
> > Miroslav Benes <mbenes@...e.cz> wrote:
> > 
> >   
> > > I agree it is kind of shooting oneself in the foot bug, because explicit 
> > > call to a sleeping function may not be the brightest thing to do. However 
> > > I see two (closely related) issues with this.
> > > 
> > > 1. It is a change in behaviour. Ftrace silently relies on an atomicity of 
> > > ops->func(). I don't see it documented anywhere, but it did not matter 
> > > because the atomicity was always guaranteed as described above. Now there 
> > > is a possibility to achieve a situation which breaks the assumption. It 
> > > makes me worried.  
> > 
> > Why? It's something that a kernel developer should be aware of. I mean,
> > that ops->func can easily be called from *any* context, like irq,
> > softirq, or even an NMI. One who hooks into any function of the kernel
> > should understand that it has special requirements, just like we don't
> > document that you can't sleep in an NMI.
> > 
> > And if you only hook to functions that can sleep, then great! You are
> > allowed to do that too. Just like calling a module function that can
> > sleep. You need to make sure nothing is calling your function when you
> > unload the module. I don't see anything that is deceptive here.  
> 
> At least the comment in ftrace_shutdown() is deceptive.

Which comment? It may require an update to be less "deceptive".

-- Steve

> 
> But well, I understood your opinion from the first reply. I just didn't 
> agree with it and that's why I expressed it. 
>