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Message-ID: <aD3DM4elo_Xt82LE@mini-arch> Date: Mon, 2 Jun 2025 08:28:51 -0700 From: Stanislav Fomichev <stfomichev@...il.com> To: Eryk Kubanski <e.kubanski@...tner.samsung.com> Cc: "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>, "maciej.fijalkowski@...el.com" <maciej.fijalkowski@...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 06/02, Eryk Kubanski 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 > > > > > 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. XSK represents a single queue and each queue is single producer single consumer. The fact that you can dup a socket and call sendmsg from different threads/processes does not lift that restriction. I think if you add synchronization on the userspace (lock(); sendmsg(); unlock();), that should help, right?
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