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

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Message-Id: <627226E3-0AC6-4B25-A338-EA65F6C85BFF@suse.de>
Date:	Sun, 12 Apr 2009 13:32:54 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	"paulmck@...ux.vnet.ibm.com" <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

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.


>                            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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