linux-kernel - Re: [PATCH 01/12] Use mutex instead of semaphore in driver core

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Date:	Wed, 2 Jan 2008 08:58:10 +0800
From:	"Dave Young" <hidave.darkstar@...il.com>
To:	"Alan Stern" <stern@...land.harvard.edu>
Cc:	"Greg KH" <gregkh@...e.de>,
	"Peter Zijlstra" <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH 01/12] Use mutex instead of semaphore in driver core

On Dec 30, 2007 6:01 AM, Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Sat, 29 Dec 2007, Dave Young wrote:
>
> > On Dec 29, 2007 1:06 PM, Dave Young <hidave.darkstar@...il.com> wrote:
> > >
> > > On Dec 29, 2007 12:42 PM, Greg KH <gregkh@...e.de> wrote:
> > > > On Sat, Dec 29, 2007 at 10:36:49AM +0800, Dave Young wrote:
> > > > > >
> > > > > The full boot dmesg with lockdep output is out, there's one warnings in it :
> > > >
> > > > Please fix that warning before the next repost of these patches (along
> > > > with fixing the problem of them not being able to be applied and
> > > > successfully built at every point in the series...)
> > > >
> > >
> > > Ok, thanks, I will fix them and repost.
> > >
> >
> > Hi,
> > After digging the code, I feel hard to fix the lockdep warning due to
> > some misterious relationship with usb.
> >
> > Could someone help on this? thanks.
> > Add usb-devel list as cc
>
> The problem isn't specific to USB.  And you will not be able to fix it
> unless you make drastic changes to the lockdep checker.
>
> lockdep warns whenever a task acquires a mutex while holding another
> mutex of the same kind (that is, the same member in another structure
> of the same type).  But there are lots of places where the kernel needs
> to acquire dev->sem for one device while already holding
> dev->parent->sem.  There's no way to remove these, which means there's
> no way to prevent lockdep from issuing a warning.
>
> Around a month ago I had a discussion with Peter Zijlstra about the
> problems in converting the device semaphores to mutexes; you may be
> able to find it in the LKML archives.  Doing the conversion while
> keeping lockdep happy is a very hard problem and we were not able to
> solve it.
>

It's unfortunate to know this, I will search and read your thread.

> It's possible that you may be able to convert the semaphores in struct
> class or other structures.  But you won't succeed with struct device.
>
> Alan Stern
>
>
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