Re: adding proper O_SYNC/O_DSYNC, was Re: O_DIRECT and barriers

From: Ulrich Drepper
Date: Thu Aug 27 2009 - 13:24:48 EST

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.

â Ulrich Drepper â Red Hat, Inc. â 444 Castro St â Mountain View, CA â
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at