Implement the async (really, delayed-write) mount option for msdosfs.

This is much simpler than for ffs since there are many fewer places
where we need to choose between a delayed write and a sync write --
just 5 in msdosfs and more than 30 in ffs.

This is more complete and correct than in ffs.  Several places in ffs
are are still missing the choice.  ffs_update() has a layering violation
that breaks callers which want to force a sync update (mainly fsync(2)
and O_SYNC write(2)).

However, fsync(2) and O_SYNC write(2) are still more broken than in
ffs, since they are broken for default (non-sync non-async) mounts
too.  Both fail to sync the FAT in all cases, and both fail to sync
the directory entry in some cases after losing a race.  Async everything
is probably safer than the half-baked sync of metadata given by default
mounts.

1 files changed, 2 insertions, 2 deletions

diff --git a/sys/fs/msdosfs/msdosfs_denode.c b/sys/fs/msdosfs/msdosfs_denode.c
index 37f1433..e3b962a 100644
--- a/sys/fs/msdosfs/msdosfs_denode.c
+++ b/sys/fs/msdosfs/msdosfs_denode.c
@@ -429,7 +429,7 @@ detrunc(dep, length, flags, cred, td)
 	if (allerror)
 		printf("detrunc(): vtruncbuf error %d\n", allerror);
 #endif
-	error = deupdat(dep, 1);
+	error = deupdat(dep, !(DETOV(dep)->v_mount->mnt_flag & MNT_ASYNC));
 	if (error != 0 && allerror == 0)
 		allerror = error;
 #ifdef MSDOSFS_DEBUG
@@ -508,7 +508,7 @@ deextend(dep, length, cred)
 	}
 	dep->de_FileSize = length;
 	dep->de_flag |= DE_UPDATE | DE_MODIFIED;
-	return (deupdat(dep, 1));
+	return (deupdat(dep, !(DETOV(dep)->v_mount->mnt_flag & MNT_ASYNC)));
 }
 
 /*