linux-kernel - Re: Linux 3.19-rc5

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Date:	Sat, 31 Jan 2015 10:16:03 +0100
From:	Bruno Prémont <bonbons@...ux-vserver.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ilya Dryomov <ilya.dryomov@...tank.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: Linux 3.19-rc5

On Thu, 29 Jan 2015 17:25:07 Linus Torvalds wrote:
> On Thu, Jan 29, 2015 at 5:12 PM, Linus Torvalds wrote:
> >
> > The WARN() was already changed to a WARN_ONCE().
> 
> Oh, but I notice that the "__set_current_state(TASK_RUNNING) ends up
> always happening.
> 
> So I think the right fix is to:
> 
>  - warn once like we do
> 
>  - but *not* do that __set_current_state() which was always total
> crap anyway
> 
> Why do I say "total crap"? Because of two independent issues:
> 
>  (a) it actually changes behavior for a debug vs non-debug kernel,
> which is a really bad idea to begin with
> 
>  (b) it's really wrong. The whole "nested sleep" case was never a
> major bug to begin with, just a possible inefficiency where constant
> nested sleeps would possibly make the outer sleep not sleep. But that
> "could possibly make" case was the unlikely case, and the debug patch
> made it happen *all* the time by explicitly setting things running.
> 
> So I think the proper patch is the attached.
> 
> The comment is also crap. The comment says
> 
>     "Blocking primitives will set (and therefore destroy)
> current->state [...]"
> 
> but the reality is that they *may* set it, and only in the unlikely
> slow-path where they actually block.
> 
> So doing this in "__may_sleep()" is just bogus and horrible horrible
> crap. It turns the "harmless ugliness" into a real *harmful* bug. The
> key word of "__may_sleep()" is that "MAY" part. It's a debug thing to
> make relatively rare cases show up.
> 
> PeterZ, please don't make "debugging" patches like this. Ever again.
> Because this was just stupid, and it took me too long to realize that
> despite the warning being shut up, the debug patch was still actively
> doing bad bad things.
> 
> Ingo, maybe you'd want to apply this through the scheduler tree, the
> way you already did the WARN_ONCE() thing.
> 
> Bruno, does this finally actually fix your pccard thing?

I will report back on Wednesday when I'm back home from FOSDEM. I don't
have the affected machine at hand at the moment.

Thanks for looking into it!
Bruno

>                               Linus

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