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

[Ulrich Drepper <drepper@redhat.com>]

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 ❖