linux-kernel - Re: [PATCH 1/1] suspend: delete sys

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Date:	Wed, 8 Jul 2015 13:17:13 +0200
From:	Pavel Machek <pavel@....cz>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Oliver Neukum <oneukum@...e.com>,
	Dave Chinner <david@...morbit.com>,
	Len Brown <lenb@...nel.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Alan Stern <stern@...land.harvard.edu>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>
Subject: Re: [PATCH 1/1] suspend: delete sys_sync()

On Tue 2015-07-07 16:32:08, Rafael J. Wysocki wrote:
> On Tuesday, July 07, 2015 03:16:48 PM Oliver Neukum wrote:
> > On Tue, 2015-07-07 at 14:14 +0200, Rafael J. Wysocki wrote:
> > > For example, on desktop systems I use user space syncs filesystems
> > > before
> > > writing to /sys/power/state, so the additional sys_sync() in the
> > > kernel doesn't
> > > seem to serve any purpose.
> > 
> > There is a race you cannot close in user space.
> 
> Yes, there is, but I'm not sure how much of a help the sync in the kernel
> provides here anyway.
> 
> Say this happens.  There is a process writing to a file running in parallel
> with the suspend process.  Suspend starts and that process is frozen.  The
> sync is called and causes all of the outstanding data to be written back.
> The user doesn't realize that the write is technically still in progress, so
> he (or she) pulls the storage device out of the system, moves it to another
> system, makes changes (say removes the file written to by the process above,
> so the blocks previously occupied by that file are now used for some metadata)
> and moves the storage back to the suspended system.  The system is resumed
> and the writing process continues writing possibly to the wrong blocks and
> corrupts the filesystem.
> 
> Is this possible?  If not, why not?

Of course it is possible; don't do it.

Some warnings, first.

 * BIG FAT WARNING
 *********************************************************
 *
 * If you touch anything on disk between suspend and resume...
 *                              ...kiss your data goodbye.

Correct option is to set up machine so that USB unplug awakes it. Or
assume USB was always removed during suspend (we actually have an
option for that).

Still, if you don't see difference from pulling USB from running
machine, and from "off" machine.. I do. And most users I know do. And
some of my machines don't indicate whether they are "off" or "sleeping".

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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