linux-kernel - Re: Race free attributes in sysfs

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Date:	Tue, 22 May 2007 17:24:15 -0400
From:	Mark Lord <lkml@....ca>
To:	Dmitry Torokhov <dtor@...ightbb.com>
Cc:	Pierre Ossman <drzeus-list@...eus.cx>, Greg KH <greg@...ah.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Race free attributes in sysfs

Dmitry Torokhov wrote:
> On Sunday 20 May 2007 07:01, Pierre Ossman wrote:
>> Hi Greg,
>>
>> I'm reworking the sysfs stuff in the MMC layer to be a bit more flexible, but
>> there is one thing that has me baffled; how do you add attributes to an object
>> in a race free manner when you have a dynamic set of attributes.
>>
>> I've looked at other parts of the kernel and they all use:
>>
>> 1. Add object.
>> 2. Add attributes.
>>
>> To me, it seems like there's a window between 1 and 2 where the object is in
>> /sys but doesn't have the proper attributes. Life for user space gets very
>> annoying if it has to add artificial delays to avoid this window.
>>
>> So how do I do this properly? Something like this would, from my point of view,
>> be the sane method:
>>
>> 1. Add hidden object.
>> 2. Add attributes.
>> 3. Show object.
>>
> 
> 1. dev->uevent_suppress = 1;
> 2. device_register(dev);
> 3. device_create_file(dev, ...);
> 4. dev->uevent_suppress = 0;
> 5. kobject_uevent(&dev->kobj, KOBJ_ADD);

I don't see how that prevents an already-running udevd
from seeing the partially filled in /sys/ entry,
if it were.. say.. already doing a /sys scan,
or even just during initial startup.

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