linux-kernel - Re: [RFC] [PATCH 0/2] Futex fault injection

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Date:	Tue, 1 Dec 2009 13:55:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Sripathi Kodi <sripathik@...ibm.com>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Darren Hart <dvhltc@...ibm.com>
Subject: Re: [RFC] [PATCH 0/2] Futex fault injection


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2009-12-01 at 11:33 +0100, Ingo Molnar wrote:
> > * Sripathi Kodi <sripathik@...ibm.com> wrote:
> > 
> > > Hi,
> > > 
> > > This patch set adds fault injection for futex subsystem. It adds 
> > > faults at places where reading/writing from user space can return 
> > > EFAULT. This will be useful in testing any significant change to futex 
> > > subsystem.
> > 
> > Instead of this unacceptably ugly and special-purpose debugfs 
> > interface, please extend perf events to allow event injection. Some 
> > other places in the kernel (which deal with rare events) want/need 
> > this capability too.
> 
> Thing is, he's using the 'normal' fault injection code to do this, I 
> see no objection to doing that.

Yes - but its impact to the futex code is butt-ugly. That some facility 
is in the kernel does not mean it gets a free pass to be applied 
everywhere and anywhere.

An example of that would be tracepoints - there's no free pass to add 
tracepoints in new places and some maintainers elect to use different 
facilities. (or reject all current facilities)

> If you want to redo the fault injection subsystem, then that's another 
> story, but then we need to convert all of its users over.

What i want to see is sane code in futex.c. If we add hooks/callbacks 
i'd like it to be a complete solution helping a lot of usecases not some 
limited approach helping testability only.

Thanks,

	Ingo
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