vfs: Implement proper O_SYNC semantics

While Linux provided an O_SYNC flag basically since day 1, it took until
Linux 2.4.0-test12pre2 to actually get it implemented for filesystems,
since that day we had generic_osync_around with only minor changes and the
great "For now, when the user asks for O_SYNC, we'll actually give
O_DSYNC" comment.  This patch intends to actually give us real O_SYNC
semantics in addition to the O_DSYNC semantics.  After Jan's O_SYNC
patches which are required before this patch it's actually surprisingly
simple, we just need to figure out when to set the datasync flag to
vfs_fsync_range and when not.

This patch renames the existing O_SYNC flag to O_DSYNC while keeping it's
numerical value to keep binary compatibility, and adds a new real O_SYNC
flag.  To guarantee backwards compatiblity it is defined as expanding to
both the O_DSYNC and the new additional binary flag (__O_SYNC) to make
sure we are backwards-compatible when compiled against the new headers.

This also means that all places that don't care about the differences can
just check O_DSYNC and get the right behaviour for O_SYNC, too - only
places that actuall care need to check __O_SYNC in addition.  Drivers and
network filesystems have been updated in a fail safe way to always do the
full sync magic if O_DSYNC is set.  The few places setting O_SYNC for
lower layers are kept that way for now to stay failsafe.

We enforce that O_DSYNC is set when __O_SYNC is set early in the open path
to make sure we always get these sane options.

Note that parisc really screwed up their headers as they already define a
O_DSYNC that has always been a no-op.  We try to repair it by using it for
the new O_DSYNC and redefinining O_SYNC to send both the traditional
O_SYNC numerical value _and_ the O_DSYNC one.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Grant Grundler <grundler@parisc-linux.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andreas Dilger <adilger@sun.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Kyle McMartin <kyle@mcmartin.ca>
Acked-by: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 30eff80fed6f..fba76fb55abf 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -43,7 +43,7 @@ static inline int uncached_access(struct file *file, unsigned long addr)
 {
 #if defined(CONFIG_IA64)
         /*
-         * On ia64, we ignore O_SYNC because we cannot tolerate memory attribute aliases.
+         * On ia64, we ignore O_DSYNC because we cannot tolerate memory attribute aliases.
          */
         return !(efi_mem_attributes(addr) & EFI_MEMORY_WB);
 #elif defined(CONFIG_MIPS)
@@ -56,9 +56,9 @@ static inline int uncached_access(struct file *file, unsigned long addr)
 #else
         /*
          * Accessing memory above the top the kernel knows about or through a file pointer
-         * that was marked O_SYNC will be done non-cached.
+         * that was marked O_DSYNC will be done non-cached.
          */
-        if (file->f_flags & O_SYNC)
+        if (file->f_flags & O_DSYNC)
                 return 1;
         return addr >= __pa(high_memory);
 #endif