linux-kernel - Re: [PATCH v2 03/10] freezer: add new freezable helpers using freezer_do_not

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Date:	Fri, 3 May 2013 10:18:17 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <tj@...nel.org>
cc:	Colin Cross <ccross@...roid.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Arve Hjønnevåg <arve@...roid.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Jeff Layton <jlayton@...hat.com>,
	Mandeep Baines <msb@...omium.org>
Subject: Re: [PATCH v2 03/10] freezer: add new freezable helpers using
 freezer_do_not_count()

On Thu, 2 May 2013, Tejun Heo wrote:

> Combined with the locking problems, I was planning to update the
> freezer such that the frozen state is implemented as a form of jobctl
> stop, so that things like ptrace / kill -9 could work on them and we
> also have the clear definition of the frozen state rather than the
> current "it may get stuck somewhere in the kernel".
> 
> But that conflicts with what you're doing here which seems pretty
> useful, so, to satisfy both goals, when somebody needs to put a
> pseudo-frozen task into the actual frozen jobctl stop, those spots
> which are currently using try_to_stop() would have to return an error,
> most likely -EINTR with TIF_SIGPENDING set, and the control should
> return towards userland so that signal handling path can be invoked.
> ie. It should be possible to steer the tasks which are considered
> frozen but not in the frozen jobctl stop into the jobctl stop without
> any side effect.  To do that, those spots basically have to be pretty
> close to the userland boundary where it can easily leave the kernel
> with -EINTR and AFAICS all the spots that you converted are like that
> (which I think is natural).  While not holding any locks doesn't
> guarantee that, I think there'd be a fairly high correlation at least
> and it'd be able to drive people towards finding out what's going on.

Don't forget about freezable kernel threads.  They never cross the 
kernel/user boundary.

Alan Stern

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