Messages in this thread | | | Date | Wed, 22 Apr 2009 14:56:20 +0200 | From | Jan Blunck <> | Subject | Re: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well |
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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.
Thanks, Jan
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