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

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Message-Id: <1200626323.5640.21.camel@lov.site>
Date:	Fri, 18 Jan 2008 04:18:43 +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 Fri, 2008-01-18 at 10:28 +0800, Dave Young wrote:
> On Jan 18, 2008 9:55 AM, Kay Sievers <kay.sievers@...y.org> wrote:
> >
> > 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.

> Then what's your opinon about the lockdep warning fix? I wonder
> whether the "soon" means we should do mutex convert after the
> class_device going away?

Yeah, might be better to wait until class_device is gone, otherwise you
may need to fix stuff that is just going to be removed. Your change to
have iterators for the class devices look like a nice preparation for
future changes though.

Our rough plan is:
 2.6.25:
  - get the ~100 patches in Greg's tree (in -mm) merged :)
 2.6.26:
  - remove the 20 char limit in "struct device"
  - get rid of "struct class_device"
 2.6.27
  - merge most of "struct class" and "struct bus_type" and have
    only one type of list and iterator for all devices of all subsystems
 2.6.27+
  - allow multiple "drivers" to bind a single device (now that the
    difference between class and bus devices is gone)

Thanks,
Kay

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