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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Date:	Mon, 30 Apr 2012 12:17:16 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.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 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.
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