linux-kernel - [PATCH 3/4] cachefiles: Fix missing clear of the CACHEFILES_OBJECT

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Date:   Thu, 05 Jul 2018 17:31:23 +0100
From:   David Howells <dhowells@...hat.com>
To:     linux-cachefs@...hat.com
Cc:     kiran.modukuri@...il.com, carmark.dlut@...il.com,
        vegard.nossum@...il.com, linux-kernel@...r.kernel.org,
        neilb@...e.com, aderobertis@...rics.net, dhowells@...hat.com,
        dja@...ens.net
Subject: [PATCH 3/4] cachefiles: Fix missing clear of the
 CACHEFILES_OBJECT_ACTIVE flag

From: KiranKumar Modukuri <kiran.modukuri@...il.com>

In cachefiles_mark_object_active(), the new object is marked active and
then we try to add it to the active object tree.  If a conflicting object
is already present, we want to wait for that to go away.  After the wait,
we go round again and try to re-mark the object as being active - but it's
already marked active from the first time we went through and a BUG is
issued.

Fix this by clearing the CACHEFILES_OBJECT_ACTIVE flag before we try again.

Analysis from Kiran Kumar Modukuri:

[Impact]
Oops during heavy NFS + FSCache + Cachefiles

CacheFiles: Error: Overlong wait for old active object to go away.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000002

CacheFiles: Error: Object already active kernel BUG at
fs/cachefiles/namei.c:163!

[Cause]
In a heavily loaded system with big files being read and truncated, an
fscache object for a cookie is being dropped and a new object being
looked. The new object being looked for has to wait for the old object
to go away before the new object is moved to active state.

[Fix]
Clear the flag 'CACHEFILES_OBJECT_ACTIVE' for the new object when
retrying the object lookup.

[Testcase]
Have run ~100 hours of NFS stress tests and have not seen this bug recur.

[Regression Potential]
 - Limited to fscache/cachefiles.

Signed-off-by: Kiran Kumar Modukuri <kiran.modukuri@...il.com>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/cachefiles/namei.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index ab0bbe93b398..b5d6dd72dfa0 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -186,6 +186,7 @@ static int cachefiles_mark_object_active(struct cachefiles_cache *cache,
 	 * need to wait for it to be destroyed */
 wait_for_old_object:
 	trace_cachefiles_wait_active(object, dentry, xobject);
+	clear_bit(CACHEFILES_OBJECT_ACTIVE, &object->flags);

 	if (fscache_object_is_live(&xobject->fscache)) {
 		pr_err("\n");
@@ -248,7 +249,6 @@ static int cachefiles_mark_object_active(struct cachefiles_cache *cache,
 	goto try_again;

 requeue:
-	clear_bit(CACHEFILES_OBJECT_ACTIVE, &object->flags);
 	cache->cache.ops->put_object(&xobject->fscache, cachefiles_obj_put_wait_timeo);
 	_leave(" = -ETIMEDOUT");
 	return -ETIMEDOUT;