linux-kernel - Re: [PATCH] Allow O_SYNC to be set by fcntl(F

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Message-Id: <20110407143719.4044c00d.akpm@linux-foundation.org>
Date:	Thu, 7 Apr 2011 14:37:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steve Rago <sar@...-labs.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] Allow O_SYNC to be set by fcntl(F_SETFL)

(did I ever reply to this?  I meant to ;))

On Fri, 25 Feb 2011 16:52:36 -0500
Steve Rago <sar@...-labs.com> 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@...-labs.com>
> ---
>   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.

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!

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!

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