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Date: Wed, 3 Mar 2010 17:23:56 -0500 (EST)
From: Alan Stern <stern@...land.harvard.edu>
To: Pavel Machek <pavel@....cz>
cc: Jens Axboe <jens.axboe@...cle.com>,
"Rafael J. Wysocki" <rjw@...k.pl>,
Maxim Levitsky <maximlevitsky@...il.com>,
linux-pm <linux-pm@...ts.linux-foundation.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] Is it supposed to be ok to call del_gendisk while
userspace is frozen?
On Wed, 3 Mar 2010, Pavel Machek wrote:
> Hi!
>
> > > > The reason for freezing those tasks is to avoid writebacks at random
> > > > times during a system sleep transition, when the underlying device may
> > > > already be suspended, right?
> > >
> > > It is also there to avoid inconsistency between in-filesystem data and
> > > snapshot in hibernation image.
> >
> > A good point, although in this case I think it won't matter. Writing
> > out a dirty page twice (once right after taking the snapshot and then
> > again after resuming from hibernation) will leave the disk in a correct
> > state.
>
> No, I don't think so. Have you considered all the various journalling
> systems?
>
> Definitely not in presence of I/O errors. Commit block can only be
> written after previous blocks are successfully writen to the journal.
>
> So lets see:
>
> <snapshot>
>
> Write previous block, write commit block, write more blocks
>
> <hibernation powerdown, restart>
>
> Error writing previous block (block now contains garbage), leading to
> kernel panic
>
> <restart>
>
> journalling assumptions broken: commit block is there, but previous
> blocks are not intact. Data loss.
>
> ...and that was the first I could think about. Lets not do
> this. Barriers were invented for a reason.
Very well. Then we still need a solution to the original problem:
Devices sometimes need to be unregistered during resume, but
del_gendisk() blocks on the writeback thread, which is frozen until
after the resume finishes. How do you suggest this be fixed?
Alan Stern
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