death to mnt_pinned

Rather than playing silly buggers with vfsmount refcounts, just have
acct_on() ask fs/namespace.c for internal clone of file->f_path.mnt
and replace it with said clone.  Then attach the pin to original
vfsmount.  Voila - the clone will be alive until the file gets closed,
making sure that underlying superblock remains active, etc., and
we can drop the original vfsmount, so that it's not kept busy.
If the file lives until the final mntput of the original vfsmount,
we'll notice that there's an fs_pin (one in bsd_acct_struct that
holds that file) and mnt_pin_kill() will take it out.  Since
->kill() is synchronous, we won't proceed past that point until
these files are closed (and private clones of our vfsmount are
gone), so we get the same ordering warranties we used to get.

mnt_pin()/mnt_unpin()/->mnt_pinned is gone now, and good riddance -
it never became usable outside of kernel/acct.c (and racy wrt
umount even there).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 19 insertions, 5 deletions

diff --git a/kernel/acct.c b/kernel/acct.c
index a7993a6..2e6cf818 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -154,7 +154,6 @@ static void close_work(struct work_struct *work)
 {
 	struct bsd_acct_struct *acct = container_of(work, struct bsd_acct_struct, work);
 	struct file *file = acct->file;
-	mnt_unpin(file->f_path.mnt);
 	if (file->f_op->flush)
 		file->f_op->flush(file, NULL);
 	__fput_sync(file);
@@ -196,9 +195,10 @@ static void acct_pin_kill(struct fs_pin *pin)
 static int acct_on(struct filename *pathname)
 {
 	struct file *file;
-	struct vfsmount *mnt;
+	struct vfsmount *mnt, *internal;
 	struct pid_namespace *ns = task_active_pid_ns(current);
 	struct bsd_acct_struct *acct, *old;
+	int err;
 
 	acct = kzalloc(sizeof(struct bsd_acct_struct), GFP_KERNEL);
 	if (!acct)
@@ -222,6 +222,21 @@ static int acct_on(struct filename *pathname)
 		filp_close(file, NULL);
 		return -EIO;
 	}
+	internal = mnt_clone_internal(&file->f_path);
+	if (IS_ERR(internal)) {
+		kfree(acct);
+		filp_close(file, NULL);
+		return PTR_ERR(internal);
+	}
+	err = mnt_want_write(internal);
+	if (err) {
+		mntput(internal);
+		kfree(acct);
+		filp_close(file, NULL);
+		return err;
+	}
+	mnt = file->f_path.mnt;
+	file->f_path.mnt = internal;
 
 	atomic_long_set(&acct->pin.count, 1);
 	acct->pin.kill = acct_pin_kill;
@@ -229,8 +244,6 @@ static int acct_on(struct filename *pathname)
 	acct->needcheck = jiffies;
 	acct->ns = ns;
 	mutex_init(&acct->lock);
-	mnt = file->f_path.mnt;
-	mnt_pin(mnt);
 	mutex_lock_nested(&acct->lock, 1);	/* nobody has seen it yet */
 	pin_insert(&acct->pin, mnt);
 
@@ -240,7 +253,8 @@ static int acct_on(struct filename *pathname)
 	else
 		ns->bacct = acct;
 	mutex_unlock(&acct->lock);
-	mntput(mnt); /* it's pinned, now give up active reference */
+	mnt_drop_write(mnt);
+	mntput(mnt);
 	return 0;
 }