From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sage Weil Subject: [PATCH 1/2] vfs: make real_lookup do dentry revalidation with i_mutex held Date: Thu, 19 Mar 2009 13:16:29 -0700 Message-ID: <1237493790-5665-1-git-send-email-sage@newdream.net> Cc: Sage Weil , Al Viro , Andreas Dilger , Yehuda Sadeh To: linux-fsdevel@vger.kernel.org, hch@infradead.org, akpm@linux-foundation.org Return-path: Received: from cobra.newdream.net ([66.33.216.30]:44786 "EHLO cobra.newdream.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752958AbZCSURz (ORCPT ); Thu, 19 Mar 2009 16:17:55 -0400 Sender: linux-fsdevel-owner@vger.kernel.org List-ID: real_lookup() is called by do_lookup() if dentry revalidation fails. If the cache is re-populated while waiting for i_mutex, it may find that a d_lookup() subsequently succeeds (see the "Uhhuh! Nasty case" comment). Previously, real_lookup() would drop i_mutex and do_revalidate() again. If revalidate failed _again_, however, it would give up with -ENOENT. The problem here that network file systems may be invalidating dentries via server callbacks, e.g. due to concurrent access from another client, and -ENOENT is frequently the wrong answer. This problem has been seen with both Lustre and Ceph. It seems possible to hit this case with NFS as well if the cache lifetime is very short. Instead, we should do_revalidate() while i_mutex is still held. If revalidation fails, we can move on to a ->lookup() and ensure a correct result without worrying about any subsequent races. Note that do_revalidate() is called with i_mutex held elsewhere. For example, do_filp_open(), lookup_create(), do_unlinkat(), do_rmdir(), and possibly others all take the directory i_mutex, and then -> lookup_hash -> __lookup_hash -> cached_lookup -> do_revalidate so this does not introduce any new locking rules for d_revalidate implementations. Yes, the goto is ugly. A cleanup patch follows. CC: Al Viro CC: Andreas Dilger Signed-off-by: Yehuda Sadeh Signed-off-by: Sage Weil --- fs/namei.c | 5 +++-- 1 files changed, 3 insertions(+), 2 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index c30e33d..b9e7128 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -489,6 +489,7 @@ static struct dentry * real_lookup(struct dentry * parent, struct qstr * name, s if (!result) { struct dentry *dentry; +do_the_lookup: /* Don't create child dentry for a dead directory. */ result = ERR_PTR(-ENOENT); if (IS_DEADDIR(dir)) @@ -512,12 +513,12 @@ out_unlock: * Uhhuh! Nasty case: the cache was re-populated while * we waited on the semaphore. Need to revalidate. */ - mutex_unlock(&dir->i_mutex); if (result->d_op && result->d_op->d_revalidate) { result = do_revalidate(result, nd); if (!result) - result = ERR_PTR(-ENOENT); + goto do_the_lookup; } + mutex_unlock(&dir->i_mutex); return result; } -- 1.5.6.5