linux-kernel - Re: [PATCH v4 2/3] genirq: reimplement exit_irq_thread() hook via task_work

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Date:	Tue, 17 Apr 2012 15:11:18 +0100
From:	David Howells <dhowells@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	dhowells@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexander Gordeev <agordeev@...hat.com>,
	Chris Zankel <chris@...kel.net>,
	David Smith <dsmith@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Larry Woodman <lwoodman@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] genirq: reimplement exit_irq_thread() hook via task_work_add()

Oleg Nesterov <oleg@...hat.com> wrote:

> 2. change irq_thread() to do task_queue_work(irq_thread_dtor)
>    at the start and task_work_cancel() before return.
> 
>    tracehook_notify_resume() can never play with kthreads,
>    only do_exit()->exit_task_work() can call the callback
>    and this is what we want.

Hmmm...  This seems wrong.  You're now using the hook in two distinct ways:
the primary use of that the hook is to detect that userspace is about to
resume processing (via TIF_NOTIFY_RESUME) and then you're abusing the fact
that the hook is also invoked via do_exit() to perform a clean up because
we've got to get rid of it somehow under that circumstance.

This only works for you because you're operating in a kernel thread which
doesn't have a userspace (and so will never see TIF_NOTIFY_RESUME).  However,
if someone tries that in an ordinary thread, it is liable to malfunction as
the record could be executed and deleted at some unpredictable point in the
future.

David
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