linux-kernel - Re: [PATCH] [RFC] Add support for uevents on block device idle changes

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Message-ID: <ac3eb2510911181340q51fb91dcy24ac2f14e2899a4@mail.gmail.com>
Date:	Wed, 18 Nov 2009 22:40:37 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Matthew Garrett <mjg59@...f.ucam.org>
Cc:	David Zeuthen <david@...ar.dk>, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, linux-hotplug@...r.kernel.org
Subject: Re: [PATCH] [RFC] Add support for uevents on block device idle 
	changes

On Wed, Nov 18, 2009 at 22:33, Matthew Garrett <mjg59@...f.ucam.org> wrote:
> On Wed, Nov 18, 2009 at 10:06:33PM +0100, Kay Sievers wrote:
>
>> That's true, but I think there is a significant difference between
>> polling every one or two seconds for media changes, and usually one or
>> two minutes for a disk idle. It's not that we poll in a rather hight
>> frequency, in an arbitrary interval, and check if some condition is
>> met.
>
> My use cases are on the order of a second.
>
>> I still don't think that we should add new event interfaces which are
>> single-subscriber only, and use global values for a specific user.
>> What if there will be another independent user for this, which might
>> want a different timeout? They fight over the trigger value to set in
>> sysfs?
>
> You can trivially multiplex without any additional wakeups. Something
> like devkit-disks can simply trigger on the lowest requested time and
> then schedule wakeups for subscribers who want a different timeout.

No, it can't do this race-free. And it's far from trivial. It can not
know when something changes the single global value.

Kay
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