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

[Christoph Hellwig]

On Thu, Aug 27, 2009 at 10:24:28AM -0700, Ulrich Drepper wrote:
> 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.

What about hte following variant:

 - given that our current O_SYNC really is and always has been actuall
   Posix O_DSYNC keep the numerical value and rename it to O_DSYNC in
   the headers.
 - Add a new O_SYNC definition:

	#define O_SYNC		(O_DSYNC|O_REALLY_SYNC)

   and do full O_SYNC handling in new kernels if O_REALLY_SYNC is
   present.
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