Saner locking around deactivate_super()
Make sure that s_umount is acquired *before* we drop the final
active reference; we still have the fast path (atomic_dec_unless)
and we have gotten rid of the window between the moment when
s_active hits zero and s_umount is acquired. Which simplifies
the living hell out of grab_super() and inotify pin_to_kill()
stuff.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/super.c b/fs/super.c
index bc734f8..157657b3 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -178,38 +178,15 @@
/**
- * deactivate_super - drop an active reference to superblock
- * @s: superblock to deactivate
- *
- * Drops an active reference to superblock, acquiring a temprory one if
- * there is no active references left. In that case we lock superblock,
- * tell fs driver to shut it down and drop the temporary reference we
- * had just acquired.
- */
-void deactivate_super(struct super_block *s)
-{
- struct file_system_type *fs = s->s_type;
- if (atomic_dec_and_test(&s->s_active)) {
- vfs_dq_off(s, 0);
- down_write(&s->s_umount);
- fs->kill_sb(s);
- put_filesystem(fs);
- put_super(s);
- }
-}
-
-EXPORT_SYMBOL(deactivate_super);
-
-/**
* deactivate_locked_super - drop an active reference to superblock
* @s: superblock to deactivate
*
- * Equivalent of up_write(&s->s_umount); deactivate_super(s);, except that
- * it does not unlock it until it's all over. As the result, it's safe to
- * use to dispose of new superblock on ->get_sb() failure exits - nobody
- * will see the sucker until it's all over. Equivalent using up_write +
- * deactivate_super is safe for that purpose only if superblock is either
- * safe to use or has NULL ->s_root when we unlock.
+ * Drops an active reference to superblock, converting it into a temprory
+ * one if there is no other active references left. In that case we
+ * tell fs driver to shut it down and drop the temporary reference we
+ * had just acquired.
+ *
+ * Caller holds exclusive lock on superblock; that lock is released.
*/
void deactivate_locked_super(struct super_block *s)
{
@@ -227,6 +204,24 @@
EXPORT_SYMBOL(deactivate_locked_super);
/**
+ * deactivate_super - drop an active reference to superblock
+ * @s: superblock to deactivate
+ *
+ * Variant of deactivate_locked_super(), except that superblock is *not*
+ * locked by caller. If we are going to drop the final active reference,
+ * lock will be acquired prior to that.
+ */
+void deactivate_super(struct super_block *s)
+{
+ if (!atomic_add_unless(&s->s_active, -1, 1)) {
+ down_write(&s->s_umount);
+ deactivate_locked_super(s);
+ }
+}
+
+EXPORT_SYMBOL(deactivate_super);
+
+/**
* grab_super - acquire an active reference
* @s: reference we are trying to make active
*
@@ -247,12 +242,10 @@
/* it's going away */
s->s_count++;
spin_unlock(&sb_lock);
- /* usually that'll be enough for it to die... */
+ /* wait for it to die */
down_write(&s->s_umount);
up_write(&s->s_umount);
put_super(s);
- /* ... but in case it wasn't, let's at least yield() */
- yield();
return 0;
}