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Message-ID: <CAL+tcoCAH+LN0HtL3wR3+Xw_GggMJS2JZwrKryuWo3f21YMVkA@mail.gmail.com> Date: Fri, 4 Jul 2025 23:29:08 +0800 From: Jason Xing <kerneljasonxing@...il.com> To: Maciej Fijalkowski <maciej.fijalkowski@...el.com> Cc: e.kubanski@...tner.samsung.com, Stanislav Fomichev <stfomichev@...il.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "bjorn@...nel.org" <bjorn@...nel.org>, "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>, "jonathan.lemon@...il.com" <jonathan.lemon@...il.com> Subject: Re: Re: [PATCH bpf v2] xsk: Fix out of order segment free in __xsk_generic_xmit() On Fri, Jul 4, 2025 at 8:35 PM Maciej Fijalkowski <maciej.fijalkowski@...el.com> wrote: > > On Fri, Jul 04, 2025 at 07:37:22AM +0800, Jason Xing wrote: > > On Mon, Jun 2, 2025 at 5:28 PM Eryk Kubanski > > <e.kubanski@...tner.samsung.com> wrote: > > > > > > > I'm not sure I understand what's the issue here. If you're using the > > > > same XSK from different CPUs, you should take care of the ordering > > > > yourself on the userspace side? > > > > > > It's not a problem with user-space Completion Queue READER side. > > > Im talking exclusively about kernel-space Completion Queue WRITE side. > > > > > > This problem can occur when multiple sockets are bound to the same > > > umem, device, queue id. In this situation Completion Queue is shared. > > > This means it can be accessed by multiple threads on kernel-side. > > > Any use is indeed protected by spinlock, however any write sequence > > > (Acquire write slot as writer, write to slot, submit write slot to reader) > > > isn't atomic in any way and it's possible to submit not-yet-sent packet > > > descriptors back to user-space as TX completed. > > > > > > Up untill now, all write-back operations had two phases, each phase > > > locks the spinlock and unlocks it: > > > 1) Acquire slot + Write descriptor (increase cached-writer by N + write values) > > > 2) Submit slot to the reader (increase writer by N) > > > > > > Slot submission was solely based on the timing. Let's consider situation, > > > where two different threads issue a syscall for two different AF_XDP sockets > > > that are bound to the same umem, dev, queue-id. > > > > > > AF_XDP setup: > > > > > > kernel-space > > > > > > Write Read > > > +--+ +--+ > > > | | | | > > > | | | | > > > | | | | > > > Completion | | | | Fill > > > Queue | | | | Queue > > > | | | | > > > | | | | > > > | | | | > > > | | | | > > > +--+ +--+ > > > Read Write > > > user-space > > > > > > > > > +--------+ +--------+ > > > | AF_XDP | | AF_XDP | > > > +--------+ +--------+ > > > > > > > > > > > > > > > > > > Possible out-of-order scenario: > > > > > > > > > writer cached_writer1 cached_writer2 > > > | | | > > > | | | > > > | | | > > > | | | > > > +--------------|--------|--------|--------|--------|--------|--------|----------------------------------------------+ > > > | | | | | | | | | > > > Completion Queue | | | | | | | | | > > > | | | | | | | | | > > > +--------------|--------|--------|--------|--------|--------|--------|----------------------------------------------+ > > > | | | > > > | | | > > > |-----------------| | > > > A) T1 syscall | | > > > writes 2 | | > > > descriptors |-----------------------------------| > > > B) T2 syscall writes 4 descriptors > > > > > > > Hi ALL, > > > > Since Maciej posted a related patch to fix this issue, it took me a > > little while to trace back to this thread. So here we are. > > > > > Notes: > > > 1) T1 and T2 AF_XDP sockets are two different sockets, > > > __xsk_generic_xmit will obtain two different mutexes. > > > 2) T1 and T2 can be executed simultaneously, there is no > > > critical section whatsoever between them. > > > 3) T1 and T2 will obtain Completion Queue Lock for acquire + write, > > > only slot acquire + write are under lock. > > > 4) T1 and T2 completion (skb destructor) > > > doesn't need to be the same order as A) and B). > > > 5) What if T1 fails after T2 acquires slots? > > > > What does it mean by 'fails'. Could you point out the accurate > > function you said? > > > > > cached_writer will be decreased by 2, T2 will > > > submit failed descriptors of T1 (they shall be > > > retransmitted in next TX). > > > Submission of writer will move writer by 4 slots > > > 2 of these slots have failed T1 values. Last two > > > slots of T2 will be missing, descriptor leak. > > > > I wonder why the leak problem happens? IIUC, in the > > __xsk_generic_xmit() + copy mode, xsk only tries to send the > > descriptor from its own tx ring to the driver, like virtio_net as an > > example. As you said, there are two xsks running in parallel. Why > > could T2 send the descriptors that T1 puts into the completion queue? > > __dev_direct_xmit() only passes the @skb that is built based on the > > addr from per xsk tx ring. > > I admit it is non-trivial case. > > Per my understanding before, based on Eryk's example, if T1 failed xmit > and reduced the cached_prod, T2 in its skb destructor would release two T1 > umem addresses and two T2 addrs instead of 4 T2 addrs. > > Putting this aside though, we had *correct* behavior before xsk > multi-buffer support, we should not let that change make it into kernel in > the first place. Hence my motivation to restore it.$ > > > > > Here are some maps related to the process you talked about: > > case 1) > > // T1 writes 2 descs in cq > > [--1--][--2--][-null-][-null-][-null-][-null-][-null-] > > | > > cached_prod > > > > // T1 fails because of NETDEV_TX_BUSY, and cq.cached_prod is decreased by 2. > > [-null-][-null-][-null-][-null-][-null-][-null-][-null-] > > | > > cached_prod > > > > // T2 starts to write at the first unused descs > > [--1--][--2--][--3--][--4--][-null-][-null-][-null-] > > | > > cached_prod > > So why can T2 send out the descs belonging to T1? In > > __xsk_generic_xmit(), xsk_cq_reserve_addr_locked() initialises the > > addr of acquired desc so it overwrites the invalid one previously > > owned by T1. The addr is from per xsk tx ring... I'm lost. Could you > > please share the detailed/key functions to shed more lights on this? > > Thanks in advance. > > Take another look at Eryk's example. The case he was providing was when t1 > produced smaller amount of addrs followed by t2 with bigger count. Then > due to t1 failure, t2 was providing addrs produced by t1. > > Your example talks about immediate failure of t1 whereas Eryk talked > about: > 1. t1 produces addrs to cq > 2. t2 produces addrs to cq > 3. t2 starts xmit > 4. t1 fails for some reason down in __xsk_generic_xmit() > 4a. t1 reduces cached_prod > 5. t2 completes, updates global state of cq's producer and exposing addrs > produced by t1 and misses part of addrs produced by t2 Wow, thanks for sharing your understanding on this. It's very clear and easy to understand to me. Thanks, Jason
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