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? Thanks! Daniele
