linux-kernel - Re: [2.6.27.24] Kernel coredump to a pipe is failing

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Message-ID: <alpine.LNX.2.00.0905272223080.2477@swampdragon.chaosbits.net>
Date:	Wed, 27 May 2009 22:25:50 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>, paul@...-scientist.net,
	linux-kernel@...r.kernel.org
Subject: Re: [2.6.27.24] Kernel coredump to a pipe is failing

On Wed, 27 May 2009, Andi Kleen wrote:

> On Tue, May 26, 2009 at 04:28:21PM -0700, Andrew Morton wrote:
> > On Wed, 27 May 2009 01:14:28 +0200
> > Andi Kleen <andi@...stfloor.org> wrote:
> > 
> > > On Tue, May 26, 2009 at 04:00:17PM -0700, Andrew Morton wrote:
> > > > dump_write() doesn't seem right, either.  If ->write() returns, say,
> > > > 100 then the dump should keep on going.  At present it treats this
> > > > return as an error.
> > > 
> > > I think that's correct actually. Short write typically means serious 
> > > issue like disk full or broken pipe, so stopping is good.
> > 
> > But we shouldn't assume that.  It could be that the ->write
> > implementation is perfectly able to absorb the remaining data.
> 
> Maybe in theory, but in practice that's unlikely isn't it?
> Disk is full or pipe is blocking etc.
> 
> > We should only error out of the write() returned zero or -EFOO.
> > The current code is simply buggy, but got lucky.
> 
> Maybe very pedantically, but I would argue that most programs 
> don't do what you're saying (retry on any short write) and
> it's actually not very nice to always write a loop for each write.
> 
My experience, from userspace, both with the codebases I currently work on 
professionally and my own hobby projects, is that people usually write a 
"write wrapper" function that deals with short writes, interrupted system 
call etc and then just call the wrapper so one does not have to open-code 
a loop everywhere one wants to call write() - one just calls the wrapper 
function that does the right thing.

-- 
Jesper Juhl <jj@...osbits.net>             http://www.chaosbits.net/
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