Re: [linux-pm] Is it supposed to be ok to call del_gendisk whileuserspace is frozen?

From: Alan Stern
Date: Wed Mar 03 2010 - 17:24:06 EST

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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