Re: [PATCH] io_uring: complete request via task work in case of DEFER_TASKRUN

[Pavel Begunkov <asml.silence@gmail.com>]

On 4/14/23 08:53, Ming Lei wrote:
> So far io_req_complete_post() only covers DEFER_TASKRUN by completing
> request via task work when the request is completed from IOWQ.
> 
> However, uring command could be completed from any context, and if io
> uring is setup with DEFER_TASKRUN, the command is required to be
> completed from current context, otherwise wait on IORING_ENTER_GETEVENTS
> can't be wakeup, and may hang forever.

fwiw, there is one legit exception, when the task is half dead
task_work will be executed by a kthread. It should be fine as it
locks the ctx down, but I can't help but wonder whether it's only
ublk_cancel_queue() affected or there are more places in ublk?

One more thing, cmds should not be setting issue_flags but only
forwarding what the core io_uring code passed, it'll get tons of
bugs in no time otherwise.

static void ublk_cancel_queue(struct ublk_queue *ubq)
{
     ...
     io_uring_cmd_done(io->cmd, UBLK_IO_RES_ABORT, 0,
                       IO_URING_F_UNLOCKED);
}

Can we replace it with task_work? It should be cold, and I
assume ublk_cancel_queue() doesn't assume that all requests will
put down by the end of the function as io_uring_cmd_done()
can offload it in any case.

-- 
Pavel Begunkov