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

[Alan Stern]

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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