Re: Keep getting the same buffer ID when RECV with IOSQE_BUFFER_SELECT

[Jens Axboe <axboe@kernel.dk>]

On 7/3/20 12:48 PM, Daniele Salvatore Albano wrote:
> Hi,
> 
> I have recently started to play with io_uring and liburing but I am
> facing an odd issue, of course initially I thought it was my code but
> after further investigation and testing some other code (
> https://github.com/frevib/io_uring-echo-server/tree/io-uring-op-provide-buffers
> ) I faced the same behaviour.
> 
> When using the IOSQE_BUFFER_SELECT with RECV I always get the first
> read right but all the subsequent return a buffer id different from
> what was used by the kernel.
> 
> The problem starts to happen only after io_uring_prep_provide_buffers
> is invoked to put back the buffer, the bid set is the one from cflags
>>> 16.
> 
> The logic is as follow:
> - io_uring_prep_provide_buffers + io_uring_submit + io_uring_wait_cqe
> initialize all the buffers at the beginning
> - within io_uring_for_each_cqe, when accepting a new connection a recv
> sqe is submitted with the IOSQE_BUFFER_SELECT flag
> - within io_uring_for_each_cqe, when recv a send sqe is submitted
> using as buffer the one specified in cflags >> 16
> - within io_uring_for_each_cqe, when send a provide buffers for the
> bid used to send the data and a recv sqes are submitted.
> 
> If I drop io_uring_prep_provide_buffers both in my code and in the
> code I referenced above it just works, but of course at some point
> there are no more buffers available.
> 
> To further debug the issue I reduced the amount of provided buffers
> and started to print out the entire bufferset and I noticed that after
> the first correct RECV the kernel stores the data in the first buffer
> of the group id but always returns the last buffer id.
> It is like after calling io_uring_prep_provide_buffers the information
> on the kernel side gets corrupted, I tried to follow the logic on the
> kernel side but there is nothing apparent that would make me
> understand why I am facing this behaviour.
> 
> The original author of that code told me on SO that he wrote & tested
> it on the kernel 5.6 + the provide buffers branch, I am facing this
> issue with 5.7.6, 5.8-rc1 and 5.8-rc3. The liburing library is built
> out of the branch, I didn't do too much testing with different
> versions but I tried to figure out where the issue was for the last
> week and within this period I have pulled multiple times the repo.
> 
> Any hint or suggestion?

Do you have a simple test case for this that can be run standalone?
I'll take a look, but I'd rather not spend time re-creating a test case
if you already have one.

-- 
Jens Axboe