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

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Date:	Tue, 07 Jul 2015 16:38:26 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Dave Chinner <david@...morbit.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Len Brown <len.brown@...el.com>, Len Brown <lenb@...nel.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Alan Stern <stern@...land.harvard.edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 1/1] suspend: delete sys_sync()

On Tue, 2015-07-07 at 16:32 +0200, 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

Well, in that case the user never got the feedback that the write is
finished. That is a race that always exists, like sending SIGKILL to a
running task.
What you describe is in principle unsolvable every time under
any circumstances.

> 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.

That is a tough nut. But that's not a reason to make it worse.
I'd say there's no reason not to use a secondary interface to
suspend without syncing or to extend or introduce such an interface
if the API is deficient.

	Regards
		Oliver


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