linux-kernel - Re: [PATCH net-next 03/12] ptr_ring: READ/WRITE_ONCE for __ptr_ring

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Date:   Fri, 26 Jan 2018 11:19:58 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next 03/12] ptr_ring: READ/WRITE_ONCE for
 __ptr_ring_empty



On 2018年01月26日 10:44, Michael S. Tsirkin wrote:
> On Fri, Jan 26, 2018 at 10:37:58AM +0800, Jason Wang wrote:
>>
>> On 2018年01月26日 07:36, Michael S. Tsirkin wrote:
>>> Lockless __ptr_ring_empty requires that consumer head is read and
>>> written at once, atomically. Annotate accordingly to make sure compiler
>>> does it correctly.  Switch locked callers to __ptr_ring_peek which does
>>> not support the lockless operation.
>>>
>>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>>> ---
>>>    include/linux/ptr_ring.h | 11 ++++++++---
>>>    1 file changed, 8 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
>>> index 8594c7b..9a72d8f 100644
>>> --- a/include/linux/ptr_ring.h
>>> +++ b/include/linux/ptr_ring.h
>>> @@ -196,7 +196,9 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r)
>>>     */
>>>    static inline bool __ptr_ring_empty(struct ptr_ring *r)
>>>    {
>>> -	return !__ptr_ring_peek(r);
>>> +	if (likely(r->size))
>>> +		return !r->queue[READ_ONCE(r->consumer_head)];
>>> +	return true;
>>>    }
>> So after patch 8, __ptr_ring_peek() did:
>>
>> static inline void *__ptr_ring_peek(struct ptr_ring *r)
>> {
>>      if (likely(r->size))
>>          return READ_ONCE(r->queue[r->consumer_head]);
>>      return NULL;
>> }
>>
>> Looks like a duplication.
>>
>> Thanks
> Nope - they are different.
>
> The reason is that __ptr_ring_peek does not need to read the consumer_head once
> since callers have a lock,

I get this.

>   and __ptr_ring_empty does not need to read
> the queue once since it merely compares it to 0.
>

Do this still work if it was called inside a loop?

Thanks