Re: [PATCH 2/2] io_uring: optimise io_fail_links()

[Nathan Chancellor <natechancellor@gmail.com>]

On Wed, Oct 14, 2020 at 08:44:22PM +0100, Pavel Begunkov wrote:
> Optimise put handling in __io_fail_links() after removing
> REQ_F_COMP_LOCKED.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  fs/io_uring.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index f61af4d487fd..271a016e8507 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1816,7 +1816,16 @@ static void __io_fail_links(struct io_kiocb *req)
>  		trace_io_uring_fail_link(req, link);
>  
>  		io_cqring_fill_event(link, -ECANCELED);
> -		io_put_req_deferred(link, 2);
> +
> +		/*
> +		 * It's ok to free under spinlock as they're not linked anymore,
> +		 * but avoid REQ_F_WORK_INITIALIZED because it may deadlock on
> +		 * work.fs->lock.
> +		 */
> +		if (link->flags | REQ_F_WORK_INITIALIZED)
> +			io_put_req_deferred(link, 2);
> +		else
> +			io_double_put_req(link);
>  	}
>  
>  	io_commit_cqring(ctx);
> -- 
> 2.24.0
> 

This part of commit 9c357fed168a ("io_uring: fix REQ_F_COMP_LOCKED by
killing it") causes a clang warning:

fs/io_uring.c:1816:19: warning: bitwise or with non-zero value always
evaluates to true [-Wtautological-bitwise-compare]
                if (link->flags | REQ_F_WORK_INITIALIZED)
                    ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

According to the comment, was it intended for that to be a bitwise AND
then negated to check for the absence of it? If so, wouldn't it be
clearer to flip the condition so that a negation is not necessary like
below? I can send a formal patch if my analysis is correct but if not,
feel free to fix it yourself and add

Reported-by: Nathan Chancellor <natechancellor@gmail.com>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 66c41d53a9d3..28b1a0fede3e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1813,10 +1813,10 @@ static void __io_fail_links(struct io_kiocb *req)
 		 * but avoid REQ_F_WORK_INITIALIZED because it may deadlock on
 		 * work.fs->lock.
 		 */
-		if (link->flags | REQ_F_WORK_INITIALIZED)
-			io_put_req_deferred(link, 2);
-		else
+		if (link->flags & REQ_F_WORK_INITIALIZED)
 			io_double_put_req(link);
+		else
+			io_put_req_deferred(link, 2);
 	}
 
 	io_commit_cqring(ctx);