Re: Regression in io_uring, leading to data corruption

[Timothy Pearson <tpearson@raptorengineering.com>]

----- Original Message -----
> From: "Jens Axboe" <axboe@kernel.dk>
> To: "Timothy Pearson" <tpearson@raptorengineering.com>
> Cc: "regressions" <regressions@lists.linux.dev>, "Pavel Begunkov" <asml.silence@gmail.com>
> Sent: Thursday, November 9, 2023 9:51:09 PM
> Subject: Re: Regression in io_uring, leading to data corruption

> Just to go back to basics, can you try this one? It'll do the exact same
> retry that io-wq is doing, just from the same task itself. If this
> fails, then something core is wrong. I don't think it will, or we'd see
> this on other platforms too of course. If this works, then it validates
> that it's some oddity on ppc with punting this operation to a thread off
> this main task.
> 
> diff --git a/io_uring/rw.c b/io_uring/rw.c
> index 64390d4e20c1..1d760570df04 100644
> --- a/io_uring/rw.c
> +++ b/io_uring/rw.c
> @@ -968,7 +968,7 @@ int io_read_mshot(struct io_kiocb *req, unsigned int
> issue_flags)
> 	return IOU_OK;
> }
> 
> -int io_write(struct io_kiocb *req, unsigned int issue_flags)
> +static int __io_write(struct io_kiocb *req, unsigned int issue_flags)
> {
> 	struct io_rw *rw = io_kiocb_to_cmd(req, struct io_rw);
> 	struct io_rw_state __s, *s = &__s;
> @@ -1092,6 +1092,19 @@ int io_write(struct io_kiocb *req, unsigned int
> issue_flags)
> 	return ret;
> }
> 
> +int io_write(struct io_kiocb *req, unsigned int issue_flags)
> +{
> +	int ret;
> +
> +	ret = __io_write(req, issue_flags);
> +	if (ret != -EAGAIN)
> +		return ret;
> +
> +	ret = __io_write(req, issue_flags & ~IO_URING_F_NONBLOCK);
> +	WARN_ON_ONCE(ret == -EAGAIN);
> +	return ret;
> +}
> +
> void io_rw_fail(struct io_kiocb *req)
> {
> 	int res;
> 

That does indeed "fix" the corruption issue.

Where is the punting actually taking place?  I can see at least one location but if it's a general issue with the punting process I should probably apply any test mitigations to all locations, and I'm not familiar enough with the codebase to be sure I've got them all...

Thanks!