Re: [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair

[Manfred Spraul <manfred@colorfullife.com>]

On 07/03/2017 07:14 PM, Paul E. McKenney wrote:
> On Mon, Jul 03, 2017 at 10:39:49AM -0400, Alan Stern wrote:
>> On Sat, 1 Jul 2017, Manfred Spraul wrote:
>>
>>> As we want to remove spin_unlock_wait() and replace it with explicit
>>> spin_lock()/spin_unlock() calls, we can use this to simplify the
>>> locking.
>>>
>>> In addition:
>>> - Reading nf_conntrack_locks_all needs ACQUIRE memory ordering.
>>> - The new code avoids the backwards loop.
>>>
>>> Only slightly tested, I did not manage to trigger calls to
>>> nf_conntrack_all_lock().
>>>
>>> Fixes: b16c29191dc8
>>> Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
>>> Cc: <stable@vger.kernel.org>
>>> Cc: Sasha Levin <sasha.levin@oracle.com>
>>> Cc: Pablo Neira Ayuso <pablo@netfilter.org>
>>> Cc: netfilter-devel@vger.kernel.org
>>> ---
>>>   net/netfilter/nf_conntrack_core.c | 44 +++++++++++++++++++++------------------
>>>   1 file changed, 24 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
>>> index e847dba..1193565 100644
>>> --- a/net/netfilter/nf_conntrack_core.c
>>> +++ b/net/netfilter/nf_conntrack_core.c
>>> @@ -96,19 +96,24 @@ static struct conntrack_gc_work conntrack_gc_work;
>>>   
>>>   void nf_conntrack_lock(spinlock_t *lock) __acquires(lock)
>>>   {
>>> +	/* 1) Acquire the lock */
>>>   	spin_lock(lock);
>>> -	while (unlikely(nf_conntrack_locks_all)) {
>>> -		spin_unlock(lock);
>>>   
>>> -		/*
>>> -		 * Order the 'nf_conntrack_locks_all' load vs. the
>>> -		 * spin_unlock_wait() loads below, to ensure
>>> -		 * that 'nf_conntrack_locks_all_lock' is indeed held:
>>> -		 */
>>> -		smp_rmb(); /* spin_lock(&nf_conntrack_locks_all_lock) */
>>> -		spin_unlock_wait(&nf_conntrack_locks_all_lock);
>>> -		spin_lock(lock);
>>> -	}
>>> +	/* 2) read nf_conntrack_locks_all, with ACQUIRE semantics */
>>> +	if (likely(smp_load_acquire(&nf_conntrack_locks_all) == false))
>>> +		return;
>> As far as I can tell, this read does not need to have ACQUIRE
>> semantics.
>>
>> You need to guarantee that two things can never happen:
>>
>>      (1) We read nf_conntrack_locks_all == false, and this routine's
>> 	critical section for nf_conntrack_locks[i] runs after the
>> 	(empty) critical section for that lock in
>> 	nf_conntrack_all_lock().
>>
>>      (2) We read nf_conntrack_locks_all == true, and this routine's
>> 	critical section for nf_conntrack_locks_all_lock runs before
>> 	the critical section in nf_conntrack_all_lock().
I was looking at nf_conntrack_all_unlock:
There is a smp_store_release() - which memory barrier does this pair with?

nf_conntrack_all_unlock()
     <arbitrary writes>
     smp_store_release(a, false)
     spin_unlock(b);

nf_conntrack_lock()
     spin_lock(c);
     xx=read_once(a)
     if (xx==false)
         return
     <arbitrary read>

>> In fact, neither one can happen even if smp_load_acquire() is replaced
>> with READ_ONCE().  The reason is simple enough, using this property of
>> spinlocks:
>>
>> 	If critical section CS1 runs before critical section CS2 (for
>> 	the same lock) then: (a) every write coming before CS1's
>> 	spin_unlock() will be visible to any read coming after CS2's
>> 	spin_lock(), and (b) no write coming after CS2's spin_lock()
>> 	will be visible to any read coming before CS1's spin_unlock().
Does this apply? The locks are different.
>> Thus for (1), assuming the critical sections run in the order mentioned
>> above, since nf_conntrack_all_lock() writes to nf_conntrack_locks_all
>> before releasing nf_conntrack_locks[i], and since nf_conntrack_lock()
>> acquires nf_conntrack_locks[i] before reading nf_conntrack_locks_all,
>> by (a) the read will always see the write.
>>
>> Similarly for (2), since nf_conntrack_all_lock() acquires
>> nf_conntrack_locks_all_lock before writing to nf_conntrack_locks_all,
>> and since nf_conntrack_lock() reads nf_conntrack_locks_all before
>> releasing nf_conntrack_locks_all_lock, by (b) the read cannot see the
>> write.
> And the Linux kernel memory model (https://lwn.net/Articles/718628/
> and https://lwn.net/Articles/720550/) agrees with Alan.  Here is
> a litmus test, which emulates spin_lock() with xchg_acquire() and
> spin_unlock() with smp_store_release():
>
> ------------------------------------------------------------------------
>
> C C-ManfredSpraul-L1G1xchgnr.litmus
>
> (* Expected result: Never.  *)
>
> {
> }
>
> P0(int *nfcla, spinlock_t *gbl, int *gbl_held, spinlock_t *lcl, int *lcl_held)
> {
> 	/* Acquire local lock. */
> 	r10 = xchg_acquire(lcl, 1);
> 	r1 = READ_ONCE(*nfcla);
> 	if (r1) {
> 		smp_store_release(lcl, 0);
> 		r11 = xchg_acquire(gbl, 1);
> 		r12 = xchg_acquire(lcl, 1);
> 		smp_store_release(gbl, 0);
> 	}
> 	r2 = READ_ONCE(*gbl_held);
> 	WRITE_ONCE(*lcl_held, 1);
> 	WRITE_ONCE(*lcl_held, 0);
> 	smp_store_release(lcl, 0);
> }
>
> P1(int *nfcla, spinlock_t *gbl, int *gbl_held, spinlock_t *lcl, int *lcl_held)
> {
> 	/* Acquire global lock. */
> 	r10 = xchg_acquire(gbl, 1);
> 	WRITE_ONCE(*nfcla, 1);
> 	r11 = xchg_acquire(lcl, 1);
> 	smp_store_release(lcl, 0);
> 	r2 = READ_ONCE(*lcl_held);
> 	WRITE_ONCE(*gbl_held, 1);
> 	WRITE_ONCE(*gbl_held, 0);
Where is the write that resets nfcla=0?
> 	smp_store_release(gbl, 0);
> }
>
> exists
> ((0:r2=1 \/ 1:r2=1) /\ 0:r10=0 /\ 0:r11=0 /\ 0:r12=0 /\ 1:r10=0 /\ 1:r11=0)
>
> ------------------------------------------------------------------------
>
> The memory model says that the forbidden state does not happen:
[...]
> Manfred, any objections to my changing your patch as Alan suggests?
I tried to pair the memory barriers:
nf_conntrack_all_unlock() contains a smp_store_release().
What does that pair with?

--
     Manfred