linux-kernel - Re: [PATCH 2/8] PM: suspend_block: Add driver to access suspend blockers from user-space

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Date:	Mon, 3 May 2010 23:49:00 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	"Linux-pm mailing list" <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
	Jim Collar <jim.collar@...are.net>,
	"Greg Kroah-Hartman" <gregkh@...e.de>, Avi Kivity <avi@...hat.com>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Nigel Cunningham <nigel@...onice.net>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/8] PM: suspend_block: Add driver to access suspend blockers from user-space

On Monday 03 May 2010, Arve Hjønnevåg wrote:
> On Mon, May 3, 2010 at 8:03 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Sunday 02 May 2010, Alan Stern wrote:
> >> On Sun, 2 May 2010, Rafael J. Wysocki wrote:
> >>
> >> > Hmm.  It doesn't seem to be possible to create two different suspend blockers
> >> > using the same file handle.  So, what exactly is a process supposed to do to
> >> > use two suspend blockers at the same time?
> >>
> >> Open the file twice, thereby obtaining two file handles.
> >
> > Well, that's what I thought.
> >
> > I must admit I'm not really comfortable with this interface.  IMO it would
> > be better if _open() created the suspend blocker giving it a name based on
> > the name of the process that called it.  Something like
> > "<process name>_<timestamp>" might work at first sight.
> >
> > Alternatively, "<process_name><number>", where <number> is 0 for the first
> > suspend blocker created by the given process, 1 for the second one etc., also
> > seems to be doable.
> 
> I think it is important to let user-space specify the name. If a
> process uses multiple suspend blockers, it is impossible to tell what
> each one is used for if they are automatically named.

Well, the problem is the only purpose of this is user space debugging, isn't it?

Now, while I don't think it's generally bad to provide kernel interfaces
helping to debug user space, I'm not quite sure if that should be done at the
expense of the clarity of kernel-user space interfaces.

I wonder how many cases there are in which distinguishing between suspend
blockers used by the same user space task is practically relevant.

Thanks,
Rafael
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