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

[Jan Blunck]

Am 11.04.2009 um 19:49 schrieb "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx 
>:

> 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.


>                            Thanx, Paul
>
> > Signed-off-by: Jan Blunck <jblunck@xxxxxxx>
> > ---
> > lib/dec_and_lock.c |    3 +--
> > 1 files changed, 1 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
> > index a65c314..e73822a 100644
> > --- a/lib/dec_and_lock.c
> > +++ b/lib/dec_and_lock.c
> > @@ -19,11 +19,10 @@
> >  */
> > int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
> > {
> > -#ifdef CONFIG_SMP
> >    /* Subtract 1 from counter unless that drops it to 0 (ie. it was  
> > 1) */
> >    if (atomic_add_unless(atomic, -1, 1))
> >        return 0;
> > -#endif
> > +
> >    /* Otherwise do it the slow way */
> >    spin_lock(lock);
> >    if (atomic_dec_and_test(atomic))
> > --
> > 1.6.0.2
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