linux-kernel - Re: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless

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Date:	Fri, 15 Mar 2013 19:34:32 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ming Lei <tom.leiming@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Shaohua Li <shli@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive
 .patch added to -mm tree

2013/3/15 Oleg Nesterov <oleg@...hat.com>:
> On 03/15, Frederic Weisbecker wrote:
>>
>> > The lack of the barrier?
>> >
>> > I thought about this, this should be fine? atomic_add_unless() has the same
>> > "problem", but this is documented in atomic_ops.txt:
>> >
>> >         atomic_add_unless requires explicit memory barriers around the operation
>> >         unless it fails (returns 0).
>> >
>> > I thought that atomic_add_unless_negative() should have the same
>> > guarantees?
>>
>> I feel very uncomfortable with that. The memory barrier is needed
>> anyway to make sure we don't deal with a stale value of the atomic val
>> (wrt. ordering against another object).
>> The following should really be expected to work without added barrier:
>>
>> void put_object(foo *obj)
>> {
>>       if (atomic_dec_return(obj->ref) == -1)
>>           free_rcu(obj);
>> }
>>
>> bool try_get_object(foo *obj)
>> {
>>       if (atomic_add_unless_negative(obj, 1))
>>            return true;
>>       return false;
>> }
>>
>> = CPU 0 =                = CPU 1
>>                                 rcu_read_lock()
>> put_object(obj0);
>>                                 obj = rcu_derefr(obj0);
>> rcu_assign_ptr(obj0, NULL);
>
> (I guess you meant rcu_assign_ptr() then put_object())

Right.

>
>>                                 if (try_get_object(obj))
>>                                      do_something...
>>                                 else
>>                                      object is dying
>>                                 rcu_read_unlock()
>
> I must have missed something.
>
> do_something() looks fine, if atomic_add_unless_negative() succeeds
> we do have a barrier?

Ok, I guess the guarantee of a barrier in case of failure is probably
not needed. But since the only way to safely read the atomic value is
a cmpxchg like operation, I guess a barrier must be involved in any
case.

Using atomic_read() may return some stale value.

>
> Anyway, I understand that it is possible to write the code which
> won't work without the uncoditional mb().

Yeah that's my fear.

>
> My point was: should we fix atomic_add_unless() then? If not, why
> should atomic_add_unless_negative() differ?

They shouldn't differ I guess.
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