linux-kernel - Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

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Message-ID: <20150212152405.GE18578@treble.redhat.com>
Date:	Thu, 12 Feb 2015 09:24:05 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Peter Zijlstra <peterz@...radead.org>, Jiri Slaby <jslaby@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	Seth Jennings <sjenning@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>
Subject: Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

On Thu, Feb 12, 2015 at 03:08:38PM +0100, Jiri Kosina wrote:
> On Thu, 12 Feb 2015, Peter Zijlstra wrote:
> I personally am not a big fan of the task_rq_lock() public exposure 
> either. What might be generally useful though (not only for livepatching) 
> would be an API that would allow for "safe" stack dump (where "safe" means 
> that guarantee, that it wouldn't be interferred by process waking up in 
> the middle of dumping, would be provided).

In general, I think a safe stack dump is needed.  A lot of the stack
dumping in the kernel seems dangerous.  For example, it looks like doing
a `cat /proc/pid/stack` while the process is writing the stack could
easily go off into the weeds.

But I don't see how it would help the livepatch case.  What happens if
the process starts running in the to-be-patched function after we call
the "safe" dump_stack() but before switching it to the new universe?

-- 
Josh
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