superblock: move pin_sb_for_writeback() to fs/super.c

The per-sb shrinker has the same requirement as the writeback
threads of ensuring that the superblock is usable and pinned for the
time it takes to run the work. Both need to take a passive reference
to the sb, take a read lock on the s_umount lock and then only
continue if an unmount is not in progress.

pin_sb_for_writeback() does this exactly, so move it to fs/super.c
and rename it to grab_super_passive() and exporting it via
fs/internal.h for all the VFS code to be able to use.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 33 insertions, 0 deletions

diff --git a/fs/super.c b/fs/super.c
index 73ab9f9b3571..e63c754447ce 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -243,6 +243,39 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
 }
 
 /*
+ *      grab_super_passive - acquire a passive reference
+ *      @s: reference we are trying to grab
+ *
+ *      Tries to acquire a passive reference. This is used in places where we
+ *      cannot take an active reference but we need to ensure that the
+ *      superblock does not go away while we are working on it. It returns
+ *      false if a reference was not gained, and returns true with the s_umount
+ *      lock held in read mode if a reference is gained. On successful return,
+ *      the caller must drop the s_umount lock and the passive reference when
+ *      done.
+ */
+bool grab_super_passive(struct super_block *sb)
+{
+        spin_lock(&sb_lock);
+        if (list_empty(&sb->s_instances)) {
+                spin_unlock(&sb_lock);
+                return false;
+        }
+
+        sb->s_count++;
+        spin_unlock(&sb_lock);
+
+        if (down_read_trylock(&sb->s_umount)) {
+                if (sb->s_root)
+                        return true;
+                up_read(&sb->s_umount);
+        }
+
+        put_super(sb);
+        return false;
+}
+
+/*
  * Superblock locking.  We really ought to get rid of these two.
  */
 void lock_super(struct super_block * sb)