linux-kernel - Re: [RFC,PATCH 14/14] utrace core

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Message-Id: <20091214194250.15BE91DE@magilla.sf.frob.com>
Date:	Mon, 14 Dec 2009 11:42:49 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Ananth Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, utrace-devel@...hat.com
Subject: Re: [RFC,PATCH 14/14] utrace core

> On 12/14, Oleg Nesterov wrote:
> >
> > IOW, we must ensure that if ever clear TIF_NOTIFY_RESUME we must not
> > miss ->pending_attach, correct? and for this case we have mb() after
> > clear_thread_flag(). Perhaps instead we should add mb__after_clear_bit()
> > into arch/ hooks, but this needs a lot of arch-dependent changes.

Since it's utrace/tracehook code that relies on the barrier I think it
makes sense to have it in tracehook_notify_resume() or utrace_resume().
The arch requirement is having done clear_thread_flag() beforehand, so the
arch-independent code can reasonably assume whatever semantics that is
guaranteed to have.

> Cough. And I always read this "rmb" as "mb". Even when I changed
> the comment to explain that we need a barrier between clear bit
> and read flags, I didn't notice it is in fact rmb...
> 
> I guess we need the trivial fix, Roland?

You're the barrier man, send me what changes it should get.


Thanks,
Roland
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