** Tags added: cscc
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https://bugs.launchpad.net/bugs/1776277
Title:
fscache cookie refcount updated incorrectly during fscache object
allocation
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Trusty:
Fix Released
Status in linux source package in Xenial:
Fix Released
Status in linux source package in Bionic:
Fix Released
Bug description:
== SRU Justification ==
[Impact]
Oops during heavy NFS + FSCache + Cachefiles use:
kernel BUG at /build/linux-Y09MKI/linux-4.4.0/fs/fscache/internal.h:321!
kernel BUG at /build/linux-Y09MKI/linux-4.4.0/fs/fscache/cookie.c:639!
[Cause]
1)Two threads are trying to do operate on a cookie and two objects.
2a)One thread tries to unmount the filesystem and in process goes over
a huge list of objects marking them dead and deleting the objects.
cookie->usage is also decremented in following path
nfs_fscache_release_super_cookie
-> __fscache_relinquish_cookie
->__fscache_cookie_put
->BUG_ON(atomic_read(&cookie->usage) <= 0);
2b)second thread tries to lookup an object for reading data in
following path
fscache_alloc_object
1) cachefiles_alloc_object
-> fscache_object_init
-> assign cookie, but usage not bumped.
2) fscache_attach_object -> fails in cant_attach_object because the
cookie's backing object or cookie's->parent object are going away
3)fscache_put_object
-> cachefiles_put_object
->fscache_object_destroy
->fscache_cookie_put
->BUG_ON(atomic_read(&cookie->usage) <= 0);
[Fix]
Bump up the cookie usage in fscache_object_init,
when it is first being assigned a cookie atomically such that the cookie
is added and bumped up if its refcount is not zero.
remove the assignment in the attach_object.
[Testcase]
A user has run ~100 hours of NFS stress tests and not seen this bug recur.
[Regression Potential]
- Limited to fscache/cachefiles.
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