Re: [PATCH] Allow O_SYNC to be set by fcntl(F_SETFL)

[Steve Rago]

On 04/07/2011 05:37 PM, Andrew Morton wrote:
> (did I ever reply to this?  I meant to ;))
>
> On Fri, 25 Feb 2011 16:52:36 -0500
> Steve Rago<sar@xxxxxxxxxxxx>  wrote:
>
> > This has probably been a problem since day 1 (I ran into this running the 2.4 kernel years ago; finally got around to
> > fixing it).  The problem is that fcntl(fd, F_SETFL, flags|O_SYNC) appears to work, but silently ignores the O_SYNC flag.
> >    Opening the file with O_SYNC works okay, but setting it later on via fcntl doesn't work.
> >
> >
> > Signed-off-by: Steve Rago<sar@xxxxxxxxxxxx>
> > ---
> >    fs/fcntl.c |    2 +-
> >    1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/fs/fcntl.c b/fs/fcntl.c
> > index cb10261..afd233a 100644
> > --- a/fs/fcntl.c
> > +++ b/fs/fcntl.c
> > @@ -143,7 +143,7 @@ SYSCALL_DEFINE1(dup, unsigned int, fildes)
> >           return ret;
> >    }
> >
> > -#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME)
> > +#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME | O_SYNC)
>
> Does any standard say that we should do this?
> http://pubs.opengroup.org/onlinepubs/007908799/xsh/fcntl.html does, I
> guess.

It's required by the Single UNIX Specification (POSIX.1).  All other major platforms allow it to be set via fcntl.  See 
bugzilla.kernel.org bug ID #5994.

> I worry a bit that this change will surprise people.  For example, this
> person:
> http://koders.com/c/fidA34D8D5EE9AA5D0AB0F3C604678E2E935E5B0246.aspx?s=dupa
> is going to wonder why his app suddenly got a lot slower!
>
> Sadly, the kernel silently ignores invalid set bits in `arg', so we
> have no reliable way of signaling to the user that our behaviour here
> changed.
>
> I wonder if we should sync the file when someone sets O_SYNC this way.
> If we don't then there is a period during which we have an fd which has
> O_SYNC set, but it has pending unwritten data.  An O_SYNC fd should
> never be in such a state!

Why not?  If I write something in non-synchronous mode, then change the file descriptor to synchronous mode, I should 
not make any assumptions about what was written prior to this point.  If I care that much, I'll call fsync.  All that 
matters is that the operating system honors the contract as specified by the system call API.

> Ho hum.  yes, I guess we should apply the patch.  But it would have
> been better to not have screwed this up in the first place!

Agreed.  Thanks for not letting this fall through the cracks.

Steve
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