f29507ce66
When a cookie is allocated that causes fscache_object structs to be allocated, those objects are initialised with the cookie pointer, but aren't blessed with a ref on that cookie unless the attachment is successfully completed in fscache_attach_object(). If attachment fails because the parent object was dying or there was a collision, fscache_attach_object() returns without incrementing the cookie counter - but upon failure of this function, the object is released which then puts the cookie, whether or not a ref was taken on the cookie. Fix this by taking a ref on the cookie when it is assigned in fscache_object_init(), even when we're creating a root object. Analysis from Kiran Kumar: This bug has been seen in 4.4.0-124-generic #148-Ubuntu kernel BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1776277 fscache cookie ref count updated incorrectly during fscache object allocation resulting in following Oops. 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] Two threads are trying to do operate on a cookie and two objects. (1) 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); (2) A 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); [NOTE from dhowells] It's unclear as to the circumstances in which (2) can take place, given that thread (1) is in nfs_kill_super(), however a conflicting NFS mount with slightly different parameters that creates a different superblock would do it. A backtrace from Kiran seems to show that this is a possibility: kernel BUG at/build/linux-Y09MKI/linux-4.4.0/fs/fscache/cookie.c:639! ... RIP: __fscache_cookie_put+0x3a/0x40 [fscache] Call Trace: __fscache_relinquish_cookie+0x87/0x120 [fscache] nfs_fscache_release_super_cookie+0x2d/0xb0 [nfs] nfs_kill_super+0x29/0x40 [nfs] deactivate_locked_super+0x48/0x80 deactivate_super+0x5c/0x60 cleanup_mnt+0x3f/0x90 __cleanup_mnt+0x12/0x20 task_work_run+0x86/0xb0 exit_to_usermode_loop+0xc2/0xd0 syscall_return_slowpath+0x4e/0x60 int_ret_from_sys_call+0x25/0x9f [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 fscache_attach_object(). [Testcase] I have run ~100 hours of NFS stress tests and not seen this bug recur. [Regression Potential] - Limited to fscache/cachefiles. Fixes: ccc4fc3d11e9 ("FS-Cache: Implement the cookie management part of the netfs API") Signed-off-by: Kiran Kumar Modukuri <kiran.modukuri@gmail.com> Signed-off-by: David Howells <dhowells@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.