netdev - Re: [PATCH net-next v2 2/3] xsk: use atomic operations around cached

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAL+tcoA7ZMsw1f6e=3WtpoyaT53cM9ryumcxT-b40VaUfuj-jw@mail.gmail.com>
Date: Fri, 28 Nov 2025 07:48:07 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	bjorn@...nel.org, magnus.karlsson@...el.com, maciej.fijalkowski@...el.com, 
	jonathan.lemon@...il.com, sdf@...ichev.me, ast@...nel.org, 
	daniel@...earbox.net, hawk@...nel.org, john.fastabend@...il.com, 
	bpf@...r.kernel.org, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v2 2/3] xsk: use atomic operations around
 cached_prod for copy mode

On Thu, Nov 27, 2025 at 11:32 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 11/27/25 2:55 PM, Jason Xing wrote:
> > On Thu, Nov 27, 2025 at 7:35 PM Paolo Abeni <pabeni@...hat.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.

Agreed. At least so far I cannot see the benefits of using
try_cmpxchg() instead as the protected area is really small. Probably
in the future I will try a better way after successfully spotting the
contention causing the performance problem.

I'm going to add your suggested-by tag since you provide this good
idea :) Thanks!

Thanks,
Jason