Message-Id: <200509202221.j8KMLhcr032238@turing-police.cc.vt.edu>
To: John Richard Moser <nigelenki@comcast.net>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Hot-patching 
In-Reply-To: Your message of "Tue, 20 Sep 2005 18:07:17 EDT."
             <43308815.1000200@comcast.net> 
From: Valdis.Kletnieks@vt.edu
References: <43308815.1000200@comcast.net>
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On Tue, 20 Sep 2005 18:07:17 EDT, John Richard Moser said:

> These bugfixes don't typically change the exported binary interface of
> the existing functions being corrected, and so it would be feasible to
> halt all processors and execute an atomic codepath to switch the symbols
> in the running kernel to point to the replacement functions from the old
> ones.  If big functions are split up into smaller ones, as long as the
> interface is the same for all existing functions, it shouldn't matter as
> well.

I believe telco switch software has been doing patch-on-the-fly for quite
a long time.  It's a royal pain in the butt, especially if you have any
dynamic 'struct foo_ops' lurking.

And you can't just plop the code in either - let's say the fix includes "add
a state bit to the 'struct foo_ctl' to track XYZ".  Now you need to think about
the fact that there's likely kmalloc'ed struct foo_ctl's already out there
that don't know about this bit.  Hilarity ensues....

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