linux-kernel - Re: [RT] Lockdep warning on boot with 2.6.31-rc5-rt1.1

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Date:	Sat, 8 Aug 2009 17:06:04 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <williams@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"greg@...ah.com" <greg@...ah.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [RT] Lockdep warning on boot with 2.6.31-rc5-rt1.1

2009/8/8 Alan Stern <stern@...land.harvard.edu>:
> On Fri, 7 Aug 2009, Peter Zijlstra wrote:

>> It used to be that _all_ dev->sem instances were taken on suspend or
>> something like that, I think that got fixed a long while back.
>>
>> I'd have to look at what the current locking requirements for dev->sem
>> are.
>
> It is supposed to be locked whenever the driver core invokes a probe,
> remove, or PM-related callback.  Under some circumstances, the parent's
> semaphore is supposed to be locked as well.  Individual subsystems may
> have their own requirements in addition to these.
>
> The ordering requirement is: Don't try to acquire a device's lock if
> you already hold the lock for a non-ancestor device.  More generally
> (if more obscurely): If you already hold device A's lock, then don't
> try to acquire the lock for device B unless you already hold the lock
> for A & B's most recent common ancestor.
>

It seems that the following case is very common, and A and B have no
common ancestor, but we can hold device A and B's lock at the same
time, can't we?

Thanks.

device A  comes in one bus:
	device_add()
         ->bus_attach_device()
            ->device_attach():drivers/base/dd.c /*holding device A's lock*/
               ->...drv->probe()		/*sleep here some time*/

then device B comes in another bus:
	device_add()
         ->bus_attach_device()
            ->device_attach():drivers/base/dd.c /*holding device B's lock*/
               ->...drv->probe()		/*sleep here some time*/

-- 
Lei Ming
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