linux-kernel - Re: sys_write() racy for multi-threaded append?

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Date:	Tue, 13 Mar 2007 09:00:31 -0500
From:	"David M. Lloyd" <dmlloyd@...rg.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"Michael K. Edwards" <medwards.linux@...il.com>,
	Bodo Eggert <7eggert@....de>,
	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: sys_write() racy for multi-threaded append?

On Tue, 2007-03-13 at 02:24 +0000, Alan Cox wrote:
> > purports to handle short writes but has never been exercised is
> > arguably worse than code that simply bombs on short write.  So if I
> > can't shim in an induce-short-writes-randomly-on-purpose mechanism
> > during development, I don't want short writes in production, period.
> 
> Easy enough to do and gcov plus dejagnu or similar tools will let you
> coverage analyse the resulting test set and replay it.

You don't even need special tools: just change your code that says:

	foo = write(fd, mybuf, mycount);

to say (for example):

	foo = write(fd, mybuf, mycount / randomly_either_1_or_2);

Why would this need kernel support?  The average developer doesn't
really need to verify that the *kernel* works.  They just need to test
their own code paths - and in this case, they can see that foo is less
than mycount (sometimes).  The code paths don't care that it was not the
kernel that caused it.

- DML

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