Re: [PATCH 05/18] Add io_uring IO interface

On 1/28/19 6:32 PM, Jann Horn wrote:

On Tue, Jan 29, 2019 at 2:31 AM Jens Axboe [off-list ref] wrote:

quoted

On 1/28/19 6:29 PM, Jann Horn wrote:

quoted

On Mon, Jan 28, 2019 at 10:35 PM Jens Axboe [off-list ref] wrote:

quoted

The submission queue (SQ) and completion queue (CQ) rings are shared
between the application and the kernel. This eliminates the need to
copy data back and forth to submit and complete IO.

[...]

quoted

+static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx)
+{
+       struct io_kiocb *req;
+
+       /* safe to use the non tryget, as we're inside ring ref already */
+       percpu_ref_get(&ctx->refs);

Is that true? In the path io_sq_thread() -> io_submit_sqes() ->
io_submit_sqe() -> io_get_req(), I don't see anything that's already
holding a reference for you. Is the worker thread holding a reference
somewhere that I'm missing?

If the thread is alive, then the ctx is alive. Before we drop the last
ref to the ctx (and kill it), we wait for the thread to exit.

Where in __io_uring_register() are you waiting for the thread to exit
before killing it? As far as I can tell, you come straight in from
syscall context, take a mutex, and do percpu_ref_kill().

I was just referring to the regular shutdown path. You are right, for
the later io_uring_register() more care needs to be taken. I'll just
switch to the tryget, it's not like the non-try is a big cycle saver
by any stretch.

-- 
Jens Axboe

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