[PATCH] remove sb->s_files and file_list_lock usage in dquot.c

Iterate over sb->s_inodes instead of sb->s_files in add_dquot_ref.  This
reduces list search and lock hold time aswell as getting rid of one of the
few uses of file_list_lock which Ingo identified as a scalability problem.

Previously we called dq_op->initialize for every inode handing of a
writeable file that wasn't initialized before.  Now we're calling it for
every inode that has a non-zero i_writecount, aka a writeable file
descriptor refering to it.

Thanks a lot to Jan Kara for running this patch through his quota test
harness.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dquot.c

@@ -688,23 +688,27 @@ static int dqinit_needed(struct inode *inode, int type)
 /* This routine is guarded by dqonoff_mutex mutex */
 static void add_dquot_ref(struct super_block *sb, int type)
 {
-	struct list_head *p;
+	struct inode *inode;
 
 restart:
-	file_list_lock();
-	list_for_each(p, &sb->s_files) {
-		struct file *filp = list_entry(p, struct file, f_u.fu_list);
-		struct inode *inode = filp->f_path.dentry->d_inode;
-		if (filp->f_mode & FMODE_WRITE && dqinit_needed(inode, type)) {
-			struct dentry *dentry = dget(filp->f_path.dentry);
-			file_list_unlock();
+	spin_lock(&inode_lock);
+	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+		if (!atomic_read(&inode->i_writecount))
+			continue;
+		if (!dqinit_needed(inode, type))
+			continue;
+		if (inode->i_state & (I_FREEING|I_WILL_FREE))
+			continue;
+
+		__iget(inode);
+		spin_unlock(&inode_lock);
+
 		sb->dq_op->initialize(inode, type);
-			dput(dentry);
+		iput(inode);
 		/* As we may have blocked we had better restart... */
 		goto restart;
 	}
-	}
-	file_list_unlock();
+	spin_unlock(&inode_lock);
 }
 
 /* Return 0 if dqput() won't block (note that 1 doesn't necessarily mean blocking) */