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    SubjectRe: 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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