linux-kernel - Re: sync, reboot, and corrupting data [was Re: 2.6.29 -mm merge plans]

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Message-ID: <20090110222614.GF14631@elf.ucw.cz>
Date:	Sat, 10 Jan 2009 23:26:15 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: sync, reboot, and corrupting data [was Re: 2.6.29 -mm merge
	plans]

On Sat 2009-01-10 23:12:32, Andi Kleen wrote:
> On Sat, Jan 10, 2009 at 10:32:23PM +0100, Pavel Machek wrote:
> > On Sat 2009-01-10 16:07:29, Andi Kleen wrote:
> > > On Thu, Jan 08, 2009 at 02:24:55PM +0100, Pavel Machek wrote:
> > > > On Wed 2009-01-07 03:57:25, Andi Kleen wrote:
> > > > > > sys_sync B which is invoked *after* sys_sync caller A should not
> > > > > > return before A. If you didn't have a global lock, they'd tend to
> > > > > > block one another's pages anyway. I think it's OK.
> > > > > 
> > > > > It means that you cannot reboot because reboot does sync.
> > > > > What happens when the sync gets stuck somewhere on a really
> > > > > slow device?
> > > > 
> > > > And what do you propose? Silently corrupt data on the slow device?
> > > 
> > > Yes not writing is better than being unable to reboot.
> > 
> > Disagreed.
> 
> Well you're just forcing the user to press power/reset/sysrq-b which
> will pretty much guarantee data loss if anything is unwritten.

Well, ok, data loss is expected in such case. It is not expected on
"clean reboot".

> > maybe reboot utility should not call sync()...
> 
> I think it should call sync(), but have a suitable timeout.
> Never spend more than 10 seconds on the sync. And give user visible 
> feedback during the countdown.

if fork() {
	sync()
} else {
	sleep(10)
	reboot()
}

..is perfectly doable in userspace.

> One alternative would be to do it with a background thread
> (which seems to be en vogue right now anyways)
> Ok I suppose with that Nick's lock is actually ok, although
> I still don't like it very much.

Yes, I believe Nick's lock is okay and needed.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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