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Date: Mon, 16 Jan 2012 09:55:07 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Greg KH <greg@kroah.com>
Cc: Mathieu Desnoyers <compudj@krystal.dyndns.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [lttng-dev] Perf ABI (was: Re: [PATCH 09/11] sched: export
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* Greg KH <greg@kroah.com> wrote:

> So I think udev actually is a good example of how to do it 
> right, we provide proper backwards compatibility in the kernel 
> to keep userspace working.

I agree, i still have a udev system that i installed 5 years 
ago, and it's working mostly fine with current kernels.

Compatibility is a desirable property, it is something that 
preserves our users - and if done right it's almost never a big 
issue technically. If it is hindering someone then there must be 
other problems.

Of course to developers the simplest approach is always to just 
develop without regard for compatibility. The simplest form of 
that is that people write patches that work fine on their own 
systems but crash the kernel on other systems. We fix those 
bugs. Another, subtler form is when the patches work fine on 
their systems but break apps on other systems. We fix those bugs 
too.

That's why we have testing, regression tracking and maintainers, 
to control that - compatibility is just another dimension to 
'correctness', in the typical case with no inherent restrictions 
on future features and possibilities.

Thanks,

	Ingo