linux-kernel - Re: Please revert 5adc55da4a7758021bcc374904b0f8b076508a11 (PCI_MULTITHREAD

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Date:	Wed, 9 May 2007 10:09:33 -0700 (PDT)
From:	david@...g.hm
To:	Cornelia Huck <cornelia.huck@...ibm.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Adrian Bunk <bunk@...sta.de>, Greg K-H <greg@...ah.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Duncan Sands <duncan.sands@...h.u-psud.fr>
Subject: Re: Please revert 5adc55da4a7758021bcc374904b0f8b076508a11
 (PCI_MULTITHREAD_PROBE)

On Wed, 9 May 2007, Cornelia Huck wrote:

> On Wed, 9 May 2007 09:18:10 -0700 (PDT),
> david@...g.hm wrote:
>
>> I'm not worried about notification to userspace, I'm worried about devices
>> getting registered during the async stage appearing in different orders on
>> different boots due to different timeings.
>
> Ah, now I think I understand what you mean. You're talking about stuff
> like block devices or net devices, right?

they will work as examples, but I am not sure that that is the limit of 
this sort of problem

>>
>> if you can identify the device well enough to register it quickly then the
>> approach that Linus proposed works well, which is
>>
>> sync probe, identify devices, register devices
>> async initialize devices
>> wait for all async to complete
>>
>> however I'm talking about cases where you can't fully identify the devices
>> (at least not well enough to register them with the kernel) and am saying
>> that doing
>>
>> sync probe
>> async initialize device, register device
>> wait for all async to complete
>>
>> results in unpredictable device ordering, but if instead you did
>>
>> sync probe
>> async initialize device
>> sync wait for all async to complete, register device
>>
>> you can get back to predictable ordering
>
> Hm, so that sound like a case for a distinct setup() routine:
>
> 1. bus calls ->probe(), which return synchronously
> 2. bus calls ->probe_async() for all devices (optional)
> 3. bus waits for all probes to finish
> 4. bus calls ->setup() for all devices (which does the registering)

this is exactly what I've been trying to describe.

> (->setup() can but need not be sync, although it should be for your
> case)

if it's not sync then you have race conditions again

> Note that ordering is not guaranteed on hotpluggable busses anyway, and
> if you use ->setup() as a function that may or may not be called later
> on, there's no ordering guarantee either. (Unless the bus/device driver
> implements something to that effect, or you have udev rules in place.)

correct.

David Lang
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