Re: [PATCH 05/10] staging: erofs: add a full barrier in erofs_workgroup_unfreeze

[Gao Xiang <gaoxiang25@huawei.com>]

Hi Andrea,

On 2018/11/23 2:50, Andrea Parri wrote:
> On Thu, Nov 22, 2018 at 06:56:32PM +0800, Gao Xiang wrote:
>> Hi Greg,
>>
>> On 2018/11/22 18:22, Greg Kroah-Hartman wrote:
>>> Please document this memory barrier.  It does not make much sense to
>>> me...
>>
>> Because we need to make the other observers noticing the latest values modified
>> in this locking period before unfreezing the whole workgroup, one way is to use
>> a memory barrier and the other way is to use ACQUIRE and RELEASE. we selected
>> the first one.
>>
>> Hmmm...ok, I will add a simple message to explain this, but I think that is
>> plain enough for a lock...
> 
> Sympathizing with Greg's request, let me add some specific suggestions:
> 
>   1. It wouldn't hurt to indicate a pair of memory accesses which are
>      intended to be "ordered" by the memory barrier in question (yes,
>      this pair might not be unique, but you should be able to provide
>      an example).
> 
>   2. Memory barriers always come matched by other memory barriers, or
>      dependencies (it really does not make sense to talk about a full
>      barrier "in isolation"): please also indicate (an instance of) a
>      matching barrier or the matching barriers.
> 
>   3. How do the hardware threads communicate?  In the acquire/release
>      pattern you mentioned above, the load-acquire *reads from* a/the
>      previous store-release, a memory access that follows the acquire
>      somehow communicate with a memory access preceding the release...
> 
>   4. It is a good practice to include the above information within an
>      (inline) comment accompanying the added memory barrier (in fact,
>      IIRC, checkpatch.pl gives you a "memory barrier without comment"
>      warning when you omit to do so); not just in the commit message.
> 
> Hope this helps.  Please let me know if something I wrote is unclear,

Thanks for taking time on the detailed explanation. I think it is helpful for me. :)
And you are right, barriers should be in pairs, and I think I need to explain more:

255 static inline bool erofs_workgroup_get(struct erofs_workgroup *grp, int *ocnt)
256 {
257         int o;
258
259 repeat:
260         o = erofs_wait_on_workgroup_freezed(grp);
261
262         if (unlikely(o <= 0))
263                 return -1;
264
265         if (unlikely(atomic_cmpxchg(&grp->refcount, o, o + 1) != o)) <- *
266                 goto repeat;
            imply a memory barrier here
267
268         *ocnt = o;
269         return 0;
270 }

I think atomic_cmpxchg implies a memory barrier semantics when the value comparison (*) succeeds...

I don't know whether my understanding is correct, If I am wrong..please correct me, or
I need to add more detailed code comments to explain in the code?

Thanks,
Gao Xiang


> 
>   Andrea
> 
> 
>>
>> Thanks,
>> Gao Xiang
>>
>>>
>>> thanks,
>>>
>>> greg k-h