linux-kernel - Re: udev races with 'arch_register_cpu' to write 1 to /sys/devices/system/cpu/cpu1/online (which is not yet created)

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Message-ID: <20130513133128.GJ6811@phenom.dumpdata.com>
Date:	Mon, 13 May 2013 09:31:28 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Igor Mammedov <imammedo@...hat.com>, chuck.anderson@...cle.com
Cc:	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org
Subject: Re: udev races with 'arch_register_cpu' to write 1 to
 /sys/devices/system/cpu/cpu1/online (which is not yet created)

On Fri, May 10, 2013 at 06:34:34PM +0200, Igor Mammedov wrote:
> >On Mon, Apr 30, 2012 at 11:51:49AM -0400, Greg KH wrote:
> >> On Mon, Apr 30, 2012 at 11:50:18AM -0400, Greg KH wrote:
> >> > On Mon, Apr 30, 2012 at 11:36:23AM -0400, Konrad Rzeszutek Wilk wrote:
> >> > > Hey Greg,
> >> > > 
> >> > > Hoping you can help with some guidance on how to fix this.
> >> > > 
> >> > > The issue is with CPU hotplug is that when a CPU goes up
> >> > > it calls 'arch_register_cpu' which eventually calls
> >> > > register_cpu. That function does these two things:
> >> > > 
> >> > > 251         error = device_register(&cpu->dev);
> >> > > 252         if (!error && cpu->hotpluggable)
> >> > > 253                 register_cpu_control(cpu);
> >> > > 
> >> > > and the device_register creates a nice little SysFS directory:
> >> > > 
> >> > > /sys/devices/system/cpu/cpu2/ which at line 251 has the 'add' attribute
> >> > > but no 'online' attribute. udev then tries to echo 1 to the 'online'
> >> > > and it we get:
> >> > > udevd-work[2421]: error opening ATTR{/sys/devices/system/cpu/cpu2/online} for writing: No such file or directory
> >> > > 
> >> > > Line 253 creates said 'online' and at that time udev [or the system admin]
> >> > > can write 1 to 'online' and the CPU goes up.
> >> > > 
> >> > > So .. any thoughts? Is there some way to inhibit from uevent being sent
> >> > > until line 253 has run?
> >> > 
> >> > Yes.
> >> 
> >> Oh, I imagine you want to know _how_ to do it too, right?  (sorry, I
> >> couldn't resist...)
> >
> >Heh.
> >> 
> >> Make this a default attribute of the cpu device, and then it will be
> >> created by the driver core before the uevent is sent to userspace.
> >> That's what you are supposed to do in the first place, adding files "by
> >> hand" is wrong, for this very reason.
> >
> >OK, will prep up a patch shortly.
> 
> Hello Konrad,
> 
> Is there a posted/accepted patch or idea was dropped?

Hey Igor,

CC-ing here Chuck here who is looking at that. My recollection (and Chuck please
correct me if I am wrong) is that the idea Greg outlined won't work very well.
The reason is that making 'online' an attribute of 'struct dev' means that:
 1) A lot of SysFS attributes that have no notion of online/offline (say ISA bus)
    will now have.
 2) The default action item (so function) to do something based on writting/reading
    from 'online' will have to be overridden by the driver using it. Which means
    another race - we can create an SysFS attribute but the default points to something
    that does nothing.

Chuck, does that sound right?
> 
> 
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