Cluster Coherent NFS and Byte Range Locking
From Linux NFS
Cluster Coherent NFS and Byte Range Locking: Background
For some time, exporting byte-range locks to NFS has been a challenge in Linux. Support for file system locks was designed with a process model and a local file system in mind. This suggested a synchronous interface in which a process that requests a lock is either granted the lock or suspended and placed on a queue. When the lock becomes available, a suspended process is granted the lock and allowed to proceed.
This synchronous approach breaks down when the request is made by a server, e.g., LOCKD or NFSD, where threads are a scarce resource. The synchronous approach threatens to block the server process, which constitutes a disaster.
Hence an asynchronous lock request interface has emerged.
One of the complexities in making that transformation is a mechanism for queueing contending requests. The queue should be fair, not giving preference to one source of lock requests over another. Ideally, contending lock requests should be granted in the order in which they are issued. This argues for a single queue of pending requests, no matter their source of issue.
Cluster file systems exported with NFS introduce another layer of complexity: often they need to coordinate their locks with a lock manager in the back end. But back end coordination can be delayed, e.g., by inter-node communication, which poses another threat to a threaded server.
Finally, NFSv4 introduces one more layer of complexity: unlike NLM locks, which block, NFSv4 byte-range locks are non-blocking, so clients contending for a lock must poll. This raises the stakes for fair queueing, as a local process waiting for a lock will almost always acquire the contended lock before an NFSv4 client can.
NFSv4 Blocking Locks
Addressing fair queueing, the NFSv4 spec suggests that the server should maintain an ordered list of pending blocking locks. More broadly, queue fairness suggests that all lock requestors (local processes, LOCKD, and the NFSv4 server) should share such an ordered list.
- Implement a shared blocking lock fair queue
- Implement the NFSv4 server fl_notify and use the fair queue
We have written patches that change the semantics of the existing file_lock->fl_block queue to integrate it with the NFSv4 server and to make it more 'fair.' This queue holds all blocking locks in the order in which they were requested. New blockers are added to the tail.
These patches have not been reviewed by the wider kernel community. However, the effort exposed a number of spec and implementation problems for which fixes were incorporated into the Linux kernel and the new NFSv4.1 draft.
The existing fl_block semantics
When a lock becomes available, local blocked processes are awakened and contending NLM clients are issued a "grant" callback. Contending NFSv4 clients, which do not block in anticipation of a server callback, receive no notification. Instead, they repeatedly poll the server to discover whether the blocked lock is available.
In more detail, when a lock is released, the kernel traverses the lock's fl_block list and wakes each blocked requester, resulting in a 'scrum' to get the lock. The winner then places all losers on its fl_block list.
This queue is fair to contending processes in that that blockers wake in order, and it is likely that a process awakened late will find the lock already claimed. But it's not fair to LOCKD, which has to perform some bookeeping tasks before requesting the lock, which gives local processes an unfair advantage. And it is especially unfair to the NFSv4 server, which must wait for a contending client question to poll again before it can attempt to acquire the lock.
The new 'fair' fl_block semantics
We tried modifying the VFS lock code so that it grants locks to queued contenders, wakes the lucky ones whose locks succeed, and returns the others to the fl_block list. We used a kernel lock to protect the fl_block list during processing. We immediately ran into a few problems:
- Claiming the lock means calling posix_lock_file which calls kmalloc which can sleep, not possible when under a spinlock; so we'd have to use a semaphore or mutex; but
- For the purposes of mandatory lock checking, this new lock must be obtained in the read/write path to check for lock compliance, and adding a semaphore or mutex to the performance-critical read/write path is thought to be inefficient.
We investigated alternative locking schemes, however we soon identified a critical problem: an NFSv4 client that has been polling for a lock may stop polling at any time without notice. (For example, a user might grow weary of waiting for an application polling for a lock to make progress and issue an interrupt.)
Granting a lock to a client that does not want it is benign if the lock grant can be revoked. However, in some cases it may be difficult for the server to revoke the errantly granted lock, e.g., if the lock has been downgraded or coalesced with other locks. In these cases, the incorrect behavior can not be reversed with a simple unlock.
This suggests the ability to request a new kind of byte-range lock from the back end file system -- a provisional lock -- that supersedes contending lock requests, but that does not downgrade or coalesce existing posix locks. This lets us remove the lock safely and easily if the client does not return.
Our patches add this provisional lock type to the VFS lock code. After these patches, the VFS lock code again walks through the fl_block list, now applying provisional locks as it can, and waking these queued contenders. We do not upgrade the lock to a real posix byte-range lock until the contender wakes up and requests (or, optionally, cancels) the lock.
To address the concern that the contender may never return, we consider the three cases: process, NLM client, and NFSv4 client.
First, the structure of the Linux kernel guarantees that a contending process on the queue must return: a process can lose interest in a lock only through an external signal, and the kernel signal handling code removes the process from the lock queue. Similarly, an NLM client that loses interest in a lock cancels its request when it wakes up, giving LOCKD the opportunity to revoke the lock request. Finally, if an NFSv4 client loses interest in a lock, NFSD revokes the lock request after a timeout.
The provisional lock is simple enough to be applied without requiring memory allocations, which sidesteps the kernel spinlock problems described earlier.
Along the way, we identified and fixed some problems with the NFSv4 protocol:
- The NFSv4 protocol has no equivalent to the NLM "cancel" call. This means that when a client process stops blocking on a lock, the server may wait up to a lease period (typically about a minute) before giving up and allowing another waiting client to take the lock. We found a solution that is backwards compatible (and thus implementable by current NFSv4.0 clients and servers), and also added language describing this solution to the new NFSv4.1 draft
- The NFSv4 protocol has no equivalent to the "grant" call; clients must thus poll very frequently if they wish to acquire contended locks in a timely manner. However, the traditional NLM grant call is known to have problems, e.g., numerous race conditions. We therefore proposed an alternate mechanism that allows a server to notify a client of lock availability without committing the server to granting the lock to that client. Speicific language for NFSv4.1 has been proposed and met with interest, but is awaiting working group consensus.
Cluster Filesystem lock() Interface
A Linux file system is allowed to export its own lock() method, but only a few file systems bother to do so. In particular, none of the file systems exported with NFS export a private lock() method. Consequently, neither LOCKD nor NFSD attempt to use private lock() methods.
Cluster filesystems, on the other hand, do want to export a lock() method that is called by LOCKD and NFSD so that the back end can maintain a consistent view across servers. However, the current private lock() interface is unsuitable for cluster file systems.
- As before, we can't afford to block the NFSv4 server or LOCKD threads, which argues for an asynchronous interface. This is especially helpful for non-blocking locks, which do not offer the option of returning a temporary "blocked" response followed by a callback that grants the request.
- From the earlier discussion, even if the request is for a blocking lock, the file system must anticipate a return from the lock() method without having fully acquired the lock. We also need to anticipate cases where a process on the client is interrupted and the client cancels the lock.
- Design and implement an asynchronous interface to the private lock() method
- Have LOCKD and NFSD test for the presence of a private lock() method and invoke the method when it is present
Acquiring a cluster file system lock may require comminication with remote hosts. To avoid blocking lock manager threads during such communication, we allow the results to be returned asynchronously.
If a file system specific lock() invocation decides that it must block, e.g., because of a delay incurred in the course of granting a non-blocking lock request, the file system returns -EINPROGRESS. Later, the file system returns the result of the lock request through a callback registered in the lock_manager_operations struct.
An FL_CANCEL flag is added to the struct file_lock to indicate to the file system that the caller wants to cancel the provided lock.
New routines vfs_lock_file, vfs_test_lock, and vfs_cancel_lock replace posix_lock_file, posix_test_file, and posix_cancel_lock in LOCKD and the NFSv4 server. They invoke the private lock() method if it exists, otherwise they invoke the posix lock() method.
Our solution has been tested with the GPFS file system. Patches have been submitted to the Linux community, and we are responding to comments.
The lack of a consumer in the Linux kernel, e.g., a cluster file system with byte-range locking, has impeded acceptance, but with GFS2 now included in the mainline Linux kernel, we have reason for optimism.