linux-kernel - RE: [PATCH] pipe: Make a partially-satisfied blocking read wait for more

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Message-ID: <4fd200bd9df24106a6d19293a495b661@AcuMS.aculab.com>
Date:   Mon, 26 Jun 2023 09:16:15 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>
CC:     Franck Grosjean <fgrosjea@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] pipe: Make a partially-satisfied blocking read wait for
 more

From: Linus Torvalds
> Sent: 24 June 2023 00:32
> 
> On Fri, 23 Jun 2023 at 16:08, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > In fact, I'd expect that patch to fail immediately on a perfectly
> > normal program that passes a token around by doing a small write to a
> > pipe, and have the "token reader" do a bigger write.
> 
> Bigger _read_, of course.
> 
> This might be hidden by such programs typically doing a single byte
> write and a single byte read, but I could easily imagine situations
> where people actually depend on the POSIX atomicity guarantees, ie you
> write a "token packet" that might be variable-sized, and the reader
> then just does a maximally sized read, knowing that it will get a full
> packet or nothing.

There are definitely programs that just do a large read in order
to consume all the single byte 'wakeup' writes.

(The 'must check' on these reads is a right PITA.)

They ought to set the pipe non-blocking, but I suspect many
don't - because it all works anyway.

	David

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