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.

4 files changed, 18 insertions, 10 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)));
 }
 
 /*
diff --git a/sys/fs/msdosfs/msdosfs_lookup.c b/sys/fs/msdosfs/msdosfs_lookup.c
index 9ebae3e..1c9a67d 100644
--- a/sys/fs/msdosfs/msdosfs_lookup.c
+++ b/sys/fs/msdosfs/msdosfs_lookup.c
@@ -625,7 +625,9 @@ createde(dep, ddep, depp, cnp)
 
 		while (--ddep->de_fndcnt >= 0) {
 			if (!(ddep->de_fndoffset & pmp->pm_crbomask)) {
-				if ((error = bwrite(bp)) != 0)
+				if (DETOV(ddep)->v_mount->mnt_flag & MNT_ASYNC)
+					bdwrite(bp);
+				else if ((error = bwrite(bp)) != 0)
 					return error;
 
 				ddep->de_fndoffset -= sizeof(struct direntry);
@@ -653,7 +655,9 @@ createde(dep, ddep, depp, cnp)
 		}
 	}
 
-	if ((error = bwrite(bp)) != 0)
+	if (DETOV(ddep)->v_mount->mnt_flag & MNT_ASYNC)
+		bdwrite(bp);
+	else if ((error = bwrite(bp)) != 0)
 		return error;
 
 	/*
@@ -951,7 +955,9 @@ removede(pdep, dep)
 			    || ep->deAttributes != ATTR_WIN95)
 				break;
 		}
-		if ((error = bwrite(bp)) != 0)
+		if (DETOV(pdep)->v_mount->mnt_flag & MNT_ASYNC)
+			bdwrite(bp);
+		else if ((error = bwrite(bp)) != 0)
 			return error;
 	} while (!(pmp->pm_flags & MSDOSFSMNT_NOWIN95)
 	    && !(offset & pmp->pm_crbomask)
diff --git a/sys/fs/msdosfs/msdosfs_vfsops.c b/sys/fs/msdosfs/msdosfs_vfsops.c
index 8b65dd8..b456667 100644
--- a/sys/fs/msdosfs/msdosfs_vfsops.c
+++ b/sys/fs/msdosfs/msdosfs_vfsops.c
@@ -76,7 +76,7 @@
 
 /* Mount options that we support. */
 static const char *msdosfs_opts[] = {
-	"noatime", "noclusterr", "noclusterw",
+	"async", "noatime", "noclusterr", "noclusterw",
 	"export", "force", "from", "sync",
 	"cs_dos", "cs_local", "cs_win", "dirmask",
 	"gid", "kiconv", "large", "longname",
diff --git a/sys/fs/msdosfs/msdosfs_vnops.c b/sys/fs/msdosfs/msdosfs_vnops.c
index 1a4c970..96f63ff 100644
--- a/sys/fs/msdosfs/msdosfs_vnops.c
+++ b/sys/fs/msdosfs/msdosfs_vnops.c
@@ -1265,8 +1265,9 @@ abortit:
 		putushort(dotdotp->deStartCluster, dp->de_StartCluster);
 		if (FAT32(pmp))
 			putushort(dotdotp->deHighClust, dp->de_StartCluster >> 16);
-		error = bwrite(bp);
-		if (error) {
+		if (fvp->v_mount->mnt_flag & MNT_ASYNC)
+			bdwrite(bp);
+		else if ((error = bwrite(bp)) != 0) {
 			/* XXX should downgrade to ro here, fs is corrupt */
 			VOP_UNLOCK(fvp, 0, td);
 			goto bad;
@@ -1390,8 +1391,9 @@ msdosfs_mkdir(ap)
 		putushort(denp[1].deHighClust, pdep->de_StartCluster >> 16);
 	}
 
-	error = bwrite(bp);
-	if (error)
+	if (ap->a_dvp->v_mount->mnt_flag & MNT_ASYNC)
+		bdwrite(bp);
+	else if ((error = bwrite(bp)) != 0)
 		goto bad;
 
 	/*