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SubjectRe: 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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