linux-kernel - Re: [PATCH 00/11] nested sleeps, fixes and debug infrastructure

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Date:	Wed, 29 Oct 2014 15:26:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>, mingo@...nel.org,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	ilya.dryomov@...tank.com, linux-kernel@...r.kernel.org,
	Eric Paris <eparis@...hat.com>, rafael.j.wysocki@...el.com
Subject: Re: [PATCH 00/11] nested sleeps, fixes and debug infrastructure

On Wed, Oct 29, 2014 at 01:00:56AM +0100, Oleg Nesterov wrote:
> On 10/28, Peter Zijlstra wrote:

> > So I talked to Rafael yesterday and I'm going to replace all the
> > wait_event*() stuff, and I suppose also freezable_schedule() because
> > they're racy.
> >
> > The moment we call freezer_do_not_count() the freezer will ignore us,
> > this means the thread could still be running (albeit not for long) when
> > the freezer reports success.
> 
> Yes, sure. IIRC the theory was that a PF_FREEZER_SKIP will do nothing
> "wrong" wrt freezing/suspend before it actually sleeps, but I guess
> today we can't assume this.

Esp. the wait_event_freezable*() family seems suspicious in that the
cond stmt can actually result in quite a lot of code.

But see below, I don't think we have a guarantee it will _ever_ sleep.

Also, this calls schedule(); try_to_freeze() in a suitable loop that's
safe against spurious wakeups, OTOH..

> > Ideally I'll be able to kill the entire freezer_do_not_count() stuff.
> 
> Agreed... but it is not clear to me what exactly we can/should do.

.. I looked at freezable_schedule() and I'm not sure how to 'fix' that.
The problem being things like signal.c:ptrace_stop() that will actually
misbehave in the face of spurious wakeups as allowed by try_to_freeze().

Then again, freezable_schedule() isn't nearly as bad as the
wait_event_freezable() stuff because it does indeed guarantee the task
only calls schedule().

Then again, it is possible to miss these tasks and report freeze success
with a running task all the same, suppose its already woken but
preempted before freezer_count(). The for_each_process_thread() loop in
try_to_freeze_tasks() will skip over it.

And all I can come up with is horrible.. maybe for another day.
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