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Date:   Thu, 21 Sep 2023 11:17:29 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Max Kellermann <max.kellermann@ionos.com>
Cc:     dhowells@redhat.com, Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: When to lock pipe->rd_wait.lock?
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On Thu, Sep 21, 2023 at 10:05:24AM +0200, Max Kellermann wrote:
> On Thu, Sep 21, 2023 at 9:28 AM Max Kellermann <max.kellermann@ionos.com> wrote:
> > I had another look at this, and something's fishy with the code or
> > with your explanation (or I still don't get it). If there is a
> > watch_queue, pipe_write() fails early with EXDEV - writing to such a
> > pipe is simply forbidden, the code is not reachable in the presence of
> > a watch_queue, therefore locking just because there might be a
> > wait_queue does not appear to make sense?
> 
> Meanwhile I have figured that the spinlock in pipe_write() is
> obsolete. It was added by David as preparation for the notification
> feature, but the notification was finally merged, it had the EXDEV,
> and I believe it was not initially planned to implement it that way?

It was supposed to get write support most likely but never got it.
Good catch.

> So I believe the spinlock is really not necessary (anymore) and I have

For pipe_write() it isn't we still need it for pipe_read().