linux-kernel - Re: adding proper O_SYNC/O_DSYNC, was Re: O

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Message-ID: <4A96C14C.8040105@redhat.com>
Date:	Thu, 27 Aug 2009 10:24:28 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Jamie Lokier <jamie@...reable.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: adding proper O_SYNC/O_DSYNC, was Re: O_DIRECT and barriers

On 08/27/2009 10:10 AM, Christoph Hellwig wrote:
> The question is how to handle this at the libc level.  Currently glibc
> defines O_DSYNC to be O_SYNC.  We would need to update glibc to pass
> through O_DSYNC for newer kernels and make sure it falls back to O_SYNC
> for olders.  I'm not sure how feasible this is, but maybe Ulrich has
> some better ideas.

The problem with O_* extensions is that the syscall doesn't fail if the 
flag is not handled.  This is a problem in the open implementation which 
can only be fixed with a new syscall.

Why cannot just go on and say we interpret O_SYNC like O_SYNC and 
O_SYNC|O_DSYNC like O_DSYNC.  The POSIX spec explicitly requires that 
the latter handled like O_SYNC.

We could handle it by allocating two bits, only one is handled in the 
kernel.  If the O_DSYNC definition for userlevel would be different from 
the kernel definition then the kernel could interpret O_SYNC|O_DSYNC 
like O_DSYNC.  The libc would then have to translate the userlevel 
O_DSYNC into the kernel O_DSYNC.  If the libc is too old for the kernel 
and the application, the userlevel flag would be passed to the kernel 
and nothing bad happens.

The cleaner alternative is to have a sys_newopen which checks for 
unknown flags and fails in that case.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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