linux-kernel - Re: Pending splice(file -> FIFO) excludes all other FIFO operations forever (was: ... always blocks read(FIFO), regardless of O

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Message-ID: <qyohloajo5pvnql3iadez4fzgiuztmx7hgokizp546lrqw3axt@ui5s6kfizj3j>
Date:   Mon, 26 Jun 2023 18:14:51 +0200
From:   Ahelenia Ziemiańska 
        <nabijaczleweli@...ijaczleweli.xyz>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Pending splice(file -> FIFO) excludes all other FIFO operations
 forever (was: ... always blocks read(FIFO), regardless of O_NONBLOCK on read
 side?)

On Mon, Jun 26, 2023 at 05:56:28PM +0200, Christian Brauner wrote:
> I mean, one workaround would probably be poll() even with O_NONBLOCK but
> I get why that's annoying and half of a solution.
poll() doesn't really change anything since it turns into a hotloop of
.revents=POLLHUP with a disconnected writer, cf.
  https://lore.kernel.org/linux-fsdevel/jbyihkyk5dtaohdwjyivambb2gffyjs3dodpofafnkkunxq7bu@jngkdxx65pux/t/#m747e2bbd0c5cffb6baaf1955f6f8b0d97e216839
The only apparently-supported way to poll pipes under linux is to
sleep()/read(O_NONBLOCK) like in the bad old days.

And even if that was a working work-around, the fundamental problem of
./spl>fifo excluding all other access to fifo is quite unfortunate too.

> tl;dr it by splicing from a regular file to a pipe where the regular
> file in splice isn't O_NONBLOCK
(Most noticeable when the "regular file" is a tty and thus the I/O never
 completes by design.)

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