Re: [PATCH net] xsk: fix __xsk_generic_xmit() error code when cq is full

[Stanislav Fomichev]

On 02/24, Magnus Karlsson wrote:
> On Sat, 22 Feb 2025 at 10:18, Wang Liang <wangliang74@xxxxxxxxxx> wrote:
> >
> > When the cq reservation is failed, the error code is not set which is
> > initialized to zero in __xsk_generic_xmit(). That means the packet is not
> > send successfully but sendto() return ok.
> >
> > Set the error code and make xskq_prod_reserve_addr()/xskq_prod_reserve()
> > return values more meaningful when the queue is full.
> 
> Hi Wang,
> 
> I agree that this would have been a really good idea if it was
> implemented from day one, but now I do not dare to change this since
> it would be changing the uapi. Let us say you have the following quite
> common code snippet for sending a packet with AF_XDP in skb mode:
> 
> err = sendmsg();
> if (err && err != -EAGAIN && err != -EBUSY)
>     goto die_due_to_error;
> continue with code
> 
> This code would with your change go and die suddenly when the
> completion ring is full instead of working. Maybe there is a piece of
> code that cleans the completion ring after these lines of code and
> next time sendmsg() is called, the packet will get sent, so the
> application used to work.
> 
> So I say: let us not do this. But if anyone has another opinion, please share.

Can we return -EBUSY from this 'if (xsk_cq_reserve_addr_locked())' case as
well?