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: <CAHk-=widpMfo_Zh5zHqL0Ct_VNqsHiwFyLUdM8wm6NF8o6T5kQ@mail.gmail.com>
Date:   Fri, 7 Jul 2023 17:21:54 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Ahelenia Ziemiańska 
        <nabijaczleweli@...ijaczleweli.xyz>,
        Christian Brauner <brauner@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        David Howells <dhowells@...hat.com>,
        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 Fri, 7 Jul 2023 at 17:07, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> > That is, do we want SIGWINCH or SIGALRM to result in a short read?
>
> Now, that's a different issue, and is actually handled by the signal
> layer: a signal that is ignored (where "ignored" includes the case of
> "default handler") will be dropped early, exactly because we don't
> want to interrupt things like tty or pipe reads when you resize the
> window.

In case you care, it's prepare_signal() -> sig_ignored() ->
sig_task_ignored() -> sig_handler_ignored() logic that does this
short-circuiting.

And while I don't think it's required by POSIX, this was definitely a
case where lots of programs that *don't* use any signal handlers at
all are very much not expecting to see -EINTR as a return value, and
used to break exactly on things like SIGWINCH when reading from a tty
or a pipe.

But that logic goes back to before linux-1.0.

In fact, I think it goes back to at least 0.99.10 (June '93).

                 Linus