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ef1d4c8d32
Currently, check_dentry_race locks the parent inode in order to ensure
it is not running in parallel with d_splice_alias for the same inode.
(For old Linux kernel versions; see commit b0461f2d: "LINUX:
Workaround d_splice_alias/d_lookup race".)
However, it is possible to hit this area of code when the parent inode
is already locked. When someone tries to create a file, directory, or
symlink, Linux tries to lookup the dentry for the target path, to see
if it already exists. While looking up the last component of the path,
Linux locks the directory, and if it finds a dentry for the target
name, it calls d_invalidate on it while the parent directory is
locked.
For a dentry with a NULL inode, we'll then try to lock the parent
inode in check_dentry_race. But since the inode is already locked, we
will deadlock.
From a user's point of view, the hang can be reproduced by doing
something similar to:
$ mkdir dir # succeeds
$ rmdir dir
$ ls -l dir
ls: cannot access dir: No such file or directory
$ mkdir dir # hangs
To avoid this, we can just change which lock we're using to avoid
check_dentry_race/d_splice_alias from running in parallel. Instead of
locking the parent inode, introduce a new global lock (called
dentry_race_sem), and lock that in check_dentry_race and around our
d_splice_alias call. We know that those are the only two users of this
new lock, so this should avoid any such deadlocks.
This does potentially reduce performance, since all tasks that hit
check_dentry_race or d_splice_alias will take the same global lock.
However, this at least still allows us to make use of negative
dentries, and this entire code path only applies to older Linux
kernels. It could be possible to add a new lock into struct vcache
instead, but using a global lock like this commit does is much
simpler.
Change-Id: Ide0f21145c83d6fbb34c637d8a36c8cd21549940
Reviewed-on: https://gerrit.openafs.org/12868
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
AFS is a distributed file system that enables users to share and access all of the files stored in a network of computers as easily as they access the files stored on their local machines. The file system is called distributed for this exact reason: files can reside on many different machines, but are available to users on every machine. OpenAFS 1.0 was originally released by IBM under the terms of the IBM Public License 1.0 (IPL10). For details on IPL10 see the LICENSE file in this directory. The current OpenAFS distribution is licensed under a combination of the IPL10 and many other licenses as granted by the relevant copyright holders. The LICENSE file in this directory contains more details, thought it is not a comprehensive statement. See INSTALL for information about building and installing OpenAFS on various platforms. See CODING for developer information and guidelines. See NEWS for recent changes to OpenAFS.