linux-kernel - Re: [PATCH 7/7] driver-core : convert semaphore to mutex in struct class

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Date:	Fri, 18 Jan 2008 02:55:38 +0100
From:	"Kay Sievers" <kay.sievers@...y.org>
To:	"Dave Young" <hidave.darkstar@...il.com>
Cc:	"Jarek Poplawski" <jarkao2@...il.com>,
	"Alan Stern" <stern@...land.harvard.edu>,
	"Greg KH" <gregkh@...e.de>, stefanr@...6.in-berlin.de,
	"David Brownell" <david-b@...bell.net>,
	"Kernel development list" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] driver-core : convert semaphore to mutex in struct class

On Jan 18, 2008 2:42 AM, Dave Young <hidave.darkstar@...il.com> wrote:
>
> On Jan 18, 2008 7:26 AM, Jarek Poplawski <jarkao2@...il.com> wrote:
> >
> > On Thu, Jan 17, 2008 at 09:31:55PM +0100, Jarek Poplawski wrote:
> > > On Thu, Jan 17, 2008 at 02:57:36PM -0500, Alan Stern wrote:
> > > > On Thu, 17 Jan 2008, Jarek Poplawski wrote:
> > > >
> > > > > On Thu, Jan 17, 2008 at 10:16:30AM -0500, Alan Stern wrote:
> > > > > > On Thu, 17 Jan 2008, Dave Young wrote:
> > > > > >
> > > > > > > > Your meaning isn't clear.  Do you mean that your patch doesn't generate
> > > > > > > > any lockdep warnings at all?  Or do you mean that it generates a single
> > > > > > > > lockdep warning at boot time and then no more warnings afterward?
> > > > > > >
> > > > > > > I means the latter one.
> > > > > >
> > > > > > That's very bad.
> > > > > >
> > > > > > For each type of violation, lockdep only gives one error message.  So
> > > > > > the fact that you get one message at boot time and then no more doesn't
> > > > > > mean the code is almost right -- it probably means the code has lots of
> > > > > > errors and you're seeing only the first one.
> > > > >
> > > > > I hope it's better than this: lockdep really stops checking after first
> > > > > warning, but I've understood from David's description that after fixing
> > > > > this one place lockdep seems to be pleased.
> > > >
> > > > That isn't what Dave said above; he said that lockdep produces a single
> > > > warning at bootup.  If he did mention anything about one place being
> > > > fixed up or lockdep being pleased, it was a while back and I've lost
> > > > track of it.
> > > >
> > > > If I recall correctly the nature of the warning was that a method
> > > > routine for one class (called with the class's mutex held) was creating
> > > > a second class and locking that class's mutex.  In principle this is
> > > > perfectly legal and should be allowed for arbitrary depths of nesting,
> > > > even though it is the sort of thing lockdep is currently unable to
> > > > handle.
> > >
> > > You are definitely right! After first reading Dave's description I got
> > > it the same way, but after re-reading I probably was misled with this
> > > "thus"! Only now I've had a look at this warning and there is really
> > > mutex_lock_nested(). Sorry Alan!
> >
> > But, on the other hand, mutex_lock() is really mutex_lock_nested(), and
> > after second checking this lockdep warning from Jan. 3, it seems
> > impossible it was get after this patch...
> >
> > Dave, could you please answer with full sentence if there is any lockdep
> > warning possible after applying these 1-7/7 patches, and if so, attach
> > current warning? Otherwise, I'll have apologized for this everybody from
> > the list soon!
>
> After digging the class usage code again, I found that the only
> possible double lock place is the class_interface_register/unregister
> in which the class_device api could be called.
>
> The scsi and pcmcia use the class_interface api, I just found the
> warning above caused by scsi part then.
>
> So I think I will need to use mutex_lock_nesting for the
> class_device_* functions.

All "struct class_device" stuff will go away very soon, and only
"struct device" will stay.
The conversion for remaining users is already in -mm. Only SCSI and IB
are missing,
but experimental patches for these exist already.

Kay
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