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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
