Re: [PATCH net-next v2 2/3] xsk: use atomic operations around cached_prod for copy mode

[Paolo Abeni <pabeni@redhat.com>]

On 11/27/25 2:55 PM, Jason Xing wrote:
> On Thu, Nov 27, 2025 at 7:35 PM Paolo Abeni <pabeni@redhat.com> wrote:
>> On 11/25/25 9:54 AM, Jason Xing wrote:
>>> diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
>>> index 44cc01555c0b..3a023791b273 100644
>>> --- a/net/xdp/xsk_queue.h
>>> +++ b/net/xdp/xsk_queue.h
>>> @@ -402,13 +402,28 @@ static inline void xskq_prod_cancel_n(struct xsk_queue *q, u32 cnt)
>>>       q->cached_prod -= cnt;
>>>  }
>>>
>>> -static inline int xskq_prod_reserve(struct xsk_queue *q)
>>> +static inline bool xsk_cq_cached_prod_nb_free(struct xsk_queue *q)
>>>  {
>>> -     if (xskq_prod_is_full(q))
>>> +     u32 cached_prod = atomic_read(&q->cached_prod_atomic);
>>> +     u32 free_entries = q->nentries - (cached_prod - q->cached_cons);
>>> +
>>> +     if (free_entries)
>>> +             return true;
>>> +
>>> +     /* Refresh the local tail pointer */
>>> +     q->cached_cons = READ_ONCE(q->ring->consumer);
>>> +     free_entries = q->nentries - (cached_prod - q->cached_cons);
>>> +
>>> +     return free_entries ? true : false;
>>> +}
>> _If_ different CPUs can call xsk_cq_cached_prod_reserve() simultaneously
>> (as the spinlock existence suggests) the above change introduce a race:
>>
>> xsk_cq_cached_prod_nb_free() can return true when num_free == 1  on
>> CPU1, and xsk_cq_cached_prod_reserve increment cached_prod_atomic on
>> CPU2 before CPU1 completed xsk_cq_cached_prod_reserve().
> 
> I think you're right... I will give it more thought tomorrow morning.
> 
> I presume using try_cmpxchg() should work as it can detect if another
> process changes @cached_prod simultaneously. They both work similarly.
> But does it make any difference compared to spin lock? I don't have
> any handy benchmark to stably measure two xsk sharing the same umem,
> probably going to implement one.
> 
> Or like what you suggested in another thread, move that lock to struct
> xsk_queue?

I think moving the lock should be preferable: I think it makes sense
from a maintenance perspective to bundle the lock in the structure it
protects, and I hope it should make the whole patch simpler.

Cheers,

Paolo