linux-kernel - Re: [uml-devel] [PATCH] uml: flush stdout before forking

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Message-ID: <CAFLxGvz=CCv5eqJNbMKrPG5FKY89CtZVUwWE=O7d2oQRVqe9=Q@mail.gmail.com>
Date:	Sun, 10 Jan 2016 16:49:13 +0100
From:	Richard Weinberger <richard.weinberger@...il.com>
To:	Vegard Nossum <vegard.nossum@...cle.com>
Cc:	Richard Weinberger <richard@....at>, Jeff Dike <jdike@...toit.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>
Subject: Re: [uml-devel] [PATCH] uml: flush stdout before forking

On Fri, Dec 18, 2015 at 9:28 PM, Vegard Nossum <vegard.nossum@...cle.com> wrote:
> I was seeing some really weird behaviour where piping UML's output
> somewhere would cause output to get duplicated:
>
>   $ ./vmlinux | head -n 40
>   Checking that ptrace can change system call numbers...Core dump limits :
>           soft - 0
>           hard - NONE
>   OK
>   Checking syscall emulation patch for ptrace...Core dump limits :
>           soft - 0
>           hard - NONE
>   OK
>   Checking advanced syscall emulation patch for ptrace...Core dump limits :
>           soft - 0
>           hard - NONE
>   OK
>   Core dump limits :
>           soft - 0
>           hard - NONE
>
> This is because these tests do a fork() which duplicates the non-empty
> stdout buffer, then glibc flushes the duplicated buffer as each child
> exits.
>
> A simple workaround is to flush before forking.
>
> Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>

Applied and queued for stable!

-- 
Thanks,
//richard