[SAUCE][XENIAL][PATCH 1/1] [CacheFiles] Fix to handle Oops in cachefiles module during new object lookup while old object is being cleaned up

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[SAUCE][XENIAL][PATCH 1/1] [CacheFiles] Fix to handle Oops in cachefiles module during new object lookup while old object is being cleaned up

Kiran Kumar Modukuri
BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1776254

[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.
Remove the BUG() for the case where the old object is still being dropped and convert to WARN()

[Testcase]
A user has run ~100 hours of NFS stress tests and not seen this bug recur.

[Regression Potential]
 - Limited to fscache/cachefiles.

Signed-off-by: kmodukuri <mailto:[hidden email]>
---
 fs/cachefiles/namei.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c index c4b8934..b08286d 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -194,7 +194,7 @@ wait_for_old_object:
                pr_err("\n");
                pr_err("Error: Unexpected object collision\n");
                cachefiles_printk_object(object, xobject);
-               BUG();
+              WARN(true, "Unexpected object collision\n");
        }
        atomic_inc(&xobject->usage);
        write_unlock(&cache->active_lock);
@@ -247,6 +247,7 @@ wait_for_old_object:

        ASSERT(!test_bit(CACHEFILES_OBJECT_ACTIVE, &xobject->flags));

+      clear_bit(CACHEFILES_OBJECT_ACTIVE, &object->flags);
        cache->cache.ops->put_object(&xobject->fscache);
        goto try_again;

--
2.7.4
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