linux-kernel - Re: [RFC PATCH 04/10] pipe: Use head and tail pointers for the ring, not cursor and length [ver #2]

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Date:   Sun, 27 Oct 2019 10:03:35 -0400
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 04/10] pipe: Use head and tail pointers for the ring,
 not cursor and length [ver #2]

This still has signs of that earlier series:

On Wed, Oct 23, 2019 at 4:17 PM David Howells <dhowells@...hat.com> wrote:
>
>                 if (rem >= ibuf->len) {
>                         *obuf = *ibuf;
>                         ibuf->ops = NULL;
> -                       pipe->curbuf = (pipe->curbuf + 1) & (pipe->buffers - 1);
> -                       pipe->nrbufs--;
> +                       tail++;
> +                       pipe_commit_read(pipe, tail);
>                 } else {
>                         if (!pipe_buf_get(pipe, ibuf))
>                                 goto out_free;

with those odd "pipe_commit_read/write()" helpers.

They make no sense, and they don't make things more legible.

It's shorter and more obvious to just write

   pipe->head = head;

than it is to write

   pipe_commit_write(pipe, head);

Even when the addition of the notifications,  it's all under the
pipe->wait.lock, so it's all just regular assignments.

Now, if at some point it starts doing fancy lockless things, at _that_
point the updates might become more complex, but that's a potential
future thing that wouldn't be relevant for a while, and isn't a reason
to make the code more obscure now.

Hmm?

             Linus