linux-kernel - Re: [PATCH] libata, freezer: avoid block device removal while system is frozen

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Message-ID: <18258432.12ryrov20c@vostro.rjw.lan>
Date:	Wed, 18 Dec 2013 01:23:57 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Tejun Heo <tj@...nel.org>
Cc:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Jens Axboe <axboe@...nel.dk>, tomaz.solc@...lix.org,
	aaron.lu@...el.com, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH] libata, freezer: avoid block device removal while system is frozen

On Tuesday, December 17, 2013 07:36:43 AM Tejun Heo wrote:
> Hello, Rafael.
> 
> On Tue, Dec 17, 2013 at 03:38:05AM +0100, Rafael J. Wysocki wrote:
> > On Friday, December 13, 2013 03:40:34 PM Tejun Heo wrote:
> > > Hello,
> > > 
> > > So, this is the laughable workaround that I came up with.  Seriously,
> > > this is tragic.  :(
> > > 
> > > Thanks.
> > > 
> > > ------- 8< -------
> > > Freezable kthreads and workqueues are fundamentally problematic in
> > > that they effectively introduce a big kernel lock widely used in the
> > > kernel and have already been the culprit of several deadlock
> > > scenarios.  This is the latest occurrence.
> > 
> > OK, so I'm too tired now to go through all that, but I'll look at it tomorrow.
> > 
> > The rule of thumb is to get rid of freezable kernel threads in the first
> > place if possible anyway if they are causing problems to happen.
> 
> Yes, that'd be awesome.  In fact, getting rid of all kernel freezables
> in non-low-level-drivers would be a great step forward.

Agreed.

> That said, we need something easily backportable so I think we probably need
> this bandaid for immediate fix for now.

Well, we need something, but I have a couple of concerns about this particular
patch (I'll reply to it with comments shortly).

Thanks,
Rafael

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