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

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Message-ID: <20090413060243.GS6822@linux.vnet.ibm.com>
Date:	Sun, 12 Apr 2009 23:02:43 -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 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?

                            Thanx, Paul
>>
>>> Signed-off-by: Jan Blunck <jblunck@...e.de>
>>> ---
>>> 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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