Re: sendmsg fails when it 'blocks'

[Jens Axboe <axboe@kernel.dk>]

On 2/9/20 2:25 AM, Jonas Bonn wrote:
> Hi Jens,
> 
> I've been trying to use io_uring to flood a network link with UDP 
> packets.  Essentially, the program just pushes a series of sendmsg() 
> calls through the SQE ring and keeps topping it up with new calls as 
> soon as the completions come in.
> 
> When the sendmsg() calls complete immediately then everything works 
> fine; however, when the calls 'block' and get queued up in the kernel 
> then the calls return either errno 97 or 22 when they're retried through 
> the workqueue (effectively, bad address or invalid iovec length).
> 
> My gut-feeling is that there's some issue copying the msghdr struct so 
> that the call that's retried isn't exactly the same one that was 
> requested.  I looked into the kernel code a bit, but couldn't really 
> make heads or tails of it so I though I'd ask for some input while I 
> keep investigating.
> 
> I noticed that liburing has a simple test for sendmsg that sends a 
> single message; the 'punted' case doesn't seem to be tested.  Is this 
> something you've tried?
> 
> Tested on kernels 5.3 to 5.6-pre and the behaviour's pretty much the 
> same with regards to the above.

You could try and pull:

git://git.kernel.dk/linux-block io_uring-5.6

into the current 5.6-pre tree and see if it works for you.

In any case, do you have a test case for this?


-- 
Jens Axboe