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 14:56:20 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 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.

Thanks,
Jan
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