Messages in this thread | | | Date | Wed, 22 Apr 2009 07:08:57 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well |
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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@linux.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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