linux-kernel - Re: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well

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Date:	Wed, 22 Apr 2009 07:08:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jan Blunck <jblunck@...e.de>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux-Kernel Mailinglist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] atomic: Only take lock when the counter drops to zero
	on UP as well

On Wed, Apr 22, 2009 at 02:56:20PM +0200, Jan Blunck wrote:
> On Sun, Apr 12, Paul E. McKenney wrote:
> 
> > On Sun, Apr 12, 2009 at 01:32:54PM +0200, Jan Blunck wrote:
> > > Am 11.04.2009 um 19:49 schrieb "Paul E. McKenney" 
> > > <paulmck@...ux.vnet.ibm.com>:
> > >
> > >> On Fri, Apr 10, 2009 at 06:13:57PM +0200, Jan Blunck wrote:
> > >>> I think it is wrong to unconditionally take the lock before calling
> > >>> atomic_dec_and_test() in _atomic_dec_and_lock(). This will deadlock in
> > >>> situation where it is known that the counter will not reach zero (e.g. 
> > >>> holding
> > >>> another reference to the same object) but the lock is already taken.
> > >>
> > >> The thought of calling _atomic_dec_and_lock() when you already hold the
> > >> lock really really scares me.
> > >>
> > >> Could you please give an example where you need to do this?
> > >>
> > >
> > > There is a part of the union mount patches that needs to do a union_put() 
> > > (which itself includes a path_put() that uses atomic_dec_and_lock() in 
> > > mntput() ). Since it is changing the namespace I need to hold the vfsmount 
> > > lock. I know that the mnt's count > 1 since it is a parent of the mnt I'm 
> > > changing in the mount tree. I could possibly delay the union_put().
> > >
> > > In general this let's atomic_dec_and_lock() behave similar on SMP and UP. 
> > > Remember that this already works with CONFIG_SMP as before Nick's patch.
> > 
> > I asked, I guess.  ;-)
> > 
> > There is some sort of common code path, so that you cannot simply call
> > atomic_dec() when holding the lock?
> 
> If it is possible I don't want to introduce another special mntput() variant
> just for that code path.

Fair enough!!!

							Thanx, Paul
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