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

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Message-ID: <20090830174852.GF7129@shareable.org>
Date:	Sun, 30 Aug 2009 18:48:52 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.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

Ulrich Drepper wrote:
> On 08/28/2009 09:44 AM, Jamie Lokier wrote:
> >Although libc's __new_open() could have this:
> >
> >     /* Old kernels only look at O_DSYNC.  It's better than nothing. */
> >     if (flags&  O_SYNC)
> >         flags |= O_DSYNC;
> >
> >Imho, it's better to not do that, and instead have
> >
> >     #define O_SYNC          (O_DSYNC|__O_SYNC_KERNEL)
> 
> Why should it be better?  You're replacing something the compiler can do 
> with zero cost with active code.

You misread; I said the zero cost thing is better.

The only reason you might use the active code is this:

    /* Upgrade O_DSYNC to O_SYNC. */

    flags = fcntl(fd, F_GETFL, 0);
    flags = (flags | O_SYNC) & ~O_DSYNC;
    fcntl(fd, F_SETFL, flags);

I'm not sure if that should work in POSIX.

-- Jamie
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