inode: remove iprune_sem

Now that we have per-sb shrinkers with a lifecycle that is a subset
of the superblock lifecycle and can reliably detect a filesystem
being unmounted, there is not longer any race condition for the
iprune_sem to protect against. Hence we can remove it.

Signed-off-by: Dave Chinner <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/inode.c

···
 __cacheline_aligned_in_smp DEFINE_SPINLOCK(inode_wb_list_lock);
 
 /*
- * iprune_sem provides exclusion between the icache shrinking and the
- * umount path.
- *
- * We don't actually need it to protect anything in the umount path,
- * but only need to cycle through it to make sure any inode that
- * prune_icache_sb took off the LRU list has been fully torn down by the
- * time we are past evict_inodes.
- */
-static DECLARE_RWSEM(iprune_sem);
-
-/*
  * Empty aops. Can be used for the cases where the user does not
  * define any of the address_space operations.
  */
···
 	spin_unlock(&inode_sb_list_lock);
 
 	dispose_list(&dispose);
-
-	/*
-	 * Cycle through iprune_sem to make sure any inode that prune_icache_sb
-	 * moved off the list before we took the lock has been fully torn
-	 * down.
-	 */
-	down_write(&iprune_sem);
-	up_write(&iprune_sem);
 }
 
 /**
···
 	int nr_scanned;
 	unsigned long reap = 0;
 
-	down_read(&iprune_sem);
 	spin_lock(&sb->s_inode_lru_lock);
 	for (nr_scanned = nr_to_scan; nr_scanned >= 0; nr_scanned--) {
 		struct inode *inode;
···
 	spin_unlock(&sb->s_inode_lru_lock);
 
 	dispose_list(&freeable);
-	up_read(&iprune_sem);
 }
 
 static void __wait_on_freeing_inode(struct inode *inode);