linux-kernel - Re: [PATCH 8/7] net/netfilter/nf_conntrack

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Date:   Sat, 3 Sep 2016 07:33:47 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will.deacon@....com>, benh@...nel.crashing.org,
        paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 8/7] net/netfilter/nf_conntrack_core: Remove another
 memory barrier

On 09/02/2016 09:22 PM, Peter Zijlstra wrote:
> On Fri, Sep 02, 2016 at 08:35:55AM +0200, Manfred Spraul wrote:
>> On 09/01/2016 06:41 PM, Peter Zijlstra wrote:
>>> On Thu, Sep 01, 2016 at 04:30:39PM +0100, Will Deacon wrote:
>>>> On Thu, Sep 01, 2016 at 05:27:52PM +0200, Manfred Spraul wrote:
>>>>> Since spin_unlock_wait() is defined as equivalent to spin_lock();
>>>>> spin_unlock(), the memory barrier before spin_unlock_wait() is
>>>>> also not required.
>>> Note that ACQUIRE+RELEASE isn't a barrier.
>>>
>>> Both are semi-permeable and things can cross in the middle, like:
>>>
>>>
>>> 	x = 1;
>>> 	LOCK
>>> 	UNLOCK
>>> 	r = y;
>>>
>>> can (validly) get re-ordered like:
>>>
>>> 	LOCK
>>> 	r = y;
>>> 	x = 1;
>>> 	UNLOCK
>>>
>>> So if you want things ordered, as I think you do, I think the smp_mb()
>>> is still needed.
>> CPU1:
>> x=1; /* without WRITE_ONCE */
>> LOCK(l);
>> UNLOCK(l);
>> <do_semop>
>> smp_store_release(x,0)
>>
>>
>> CPU2;
>> LOCK(l)
>> if (smp_load_acquire(x)==1) goto slow_path
>> <do_semop>
>> UNLOCK(l)
>>
>> Ordering is enforced because both CPUs access the same lock.
>>
>> x=1 can't be reordered past the UNLOCK(l), I don't see that further
>> guarantees are necessary.
>>
>> Correct?
> Correct, sadly implementations do not comply :/ In fact, even x86 is
> broken here.
>
> I spoke to Will earlier today and he suggests either making
> spin_unlock_wait() stronger to avoids any and all such surprises or just
> getting rid of the thing.
>
> I'm not sure which way we should go, but please hold off on these two
> patches until I've had a chance to audit all of those implementations
> again.
For me, it doesn't really matter.
spin_unlock_wait() as "R", as "RAcq" or as "spin_lock(); spin_lock();" - 
I just want a usable definition for ipc/sem.c

So (just to keep Andrew updated):
Ready for merging is: (Bugfixes, safe just with spin_unlock_wait() as "R"):

- 45a449340cd1 ("ipc/sem.c: fix complex_count vs. simple op race")
   Cc stable, back to 3.10 ...
- 7fd5653d9986 ("net/netfilter/nf_conntrack_core: Fix memory barriers.")
   Cc stable, back to ~4.5

--
     Manfred