This chapter is from the book
This chapter is from the book
This chapter is from the book
15.6 Locking in UFS
UFS uses two basic types of locks: kmutex_t and krwlock_t. The workings of these synchronization primitives is covered in Chapter 17. UFS locks can be divided into eight categories:
- Inode locks
- Queue locks
- ACL locks
- VNODE locks
- VFS locks
- VOP_RWLOCK
- ufs_iuniqtime_lock
- Logging locks
15.6.1 UFS Lock Descriptions
Tables 15.2 through 15.9 describe the UFS locks in more detail.
Table 15.2. Inode Locks
|
Name |
Type |
Description |
|
i_rwlock |
krwlock_t |
|
|
i_contents |
krwlock_t |
|
|
i_tlock |
kmutex_t |
|
|
i_hlock |
kmutex_t |
|
Table 15.3. Inode Queue Locks
|
Name |
Type |
Description |
|
ufs_scan_lock |
kmutex_t |
|
|
ufs_q->uq_mutex |
krwlock_t |
|
|
ufs_hlock |
kmutex_t |
|
|
ih_lock |
kmutex_t |
|
Table 15.4. Quota Queue Locks
|
Name |
Type |
Description |
|
dq_cachelock |
kmutex_t |
|
|
dq_freelock |
kmutex_t |
|
|
dq_rwlock |
krwlock_t |
|
|
dqout.dq_lock |
kmutex_t |
|
Table 15.5. VNODE Locks
|
Name |
Type |
Description |
|
v_lock |
kmutex_t |
|
Table 15.6. ACL Locks
|
Name |
Type |
Description |
|
s_lock |
krwlock_t |
|
Table 15.7. VFS Locks
|
Name |
Type |
Description |
|
vfs_lock |
kmutex_t |
|
|
vfs_dqrwlock |
krwlock_t |
|
|
ufsvfs_mutex |
kmutex_t |
|
Table 15.8. VOP_RWLOCK or ufs_rwlock
|
Name |
Type |
Description |
|
ufs_rwlock() |
function |
|
Table 15.9. Logging Locks
|
Name |
Type |
Description |
|
mtm_lock |
kmutex_t |
|
|
mtm_mutex |
kmutex_t |
|
|
mtm_rwlock |
krwlock_t |
|
|
un_log_mutex |
kmutex_t |
|
|
un_state_mutex |
kmutex_t |
|
15.6.2 Inode Lock Ordering
Now that we are all familiar with the several different types of locks available in UFS, let us put them in order as if we were to work on an inode. Lock ordering is critical, and any mistake will more than likely cause the system to deadlock, and may end up panicking it!
Figure 15.16 give us a quick overview of lock ordering specific to the inode.
)
Figure 15.16 Inode Lock Ordering Precedence
15.6.3 UFS Lockfs Protocol
Along with basic inode locking, UFS also provides a mechanism to quiesce a file system for file system locking and for the forced unmounting of a file system. All VOPs (vnode operations) in UFS are required to follow the UFS lock protocol with ufs_lockfs_begin() and ufs_lockfs_end(), although the following functions purposely do not adhere to the tradition:
- ufs_close
- ufs_putpage
- ufs_inactive
- ufs_addmap
- ufs_delmap
- ufs_rwlock
- ufs_rwunlock
- ufs_poll
The basic principle here is that UFS supports various file system lock states (see list below) and each vnode operation must initiate the protocol by calling ufs_lockfs_begin() with an appropriate lock mask (a lock that this operation might grab while it is being processed) and end the protocol by calling ufs_lockfs_end before it returns. This way, UFS knows exactly how many vnode operations are in progress for the given file system by incrementing and decrementing the ul_vnops_cnt variable in the file-system-dependent ulockfs structure. If the file system is hard-locked, the thread gets an EIO error. If the file system is error-locked, then the thread is blocked.
Here are the file system locks and their actions.
- Write lock. Suspends writes that would modify the file system. Access times are not kept while a file system is write-locked.
- Name lock. Suspends accesses that could change or remove existing directories entries.
- Delete lock. Suspends access that could remove directory entries.
- Hard lock. Returns an error upon every access to the locked file system and cannot be unlocked. Hard-locked file systems can be unmounted. Hard lock supports forcible unmount.
- Error lock. Blocks all local access to the file system and returns EWOULDBLOCK on all remote access. File systems are error-locked by UFS upon detection of internal inconsistency. They can only be unlocked after successful repair by fsck, which is usually done automatically. Error-locked file systems can be unmounted. Once the file system becomes clean, it can be upgraded to a hard lock.
- Soft lock. Quiesces a file system.
- Unlock. Awakens suspended accesses, releases existing locks, and flushes the file system.
While a vnode operation is being executed in UFS, a call can be made to another vnode function on the same UFS or a different UFS. This is called recursive VOP. The per-file system vnode operation counter is not incremented or decremented during recursive calls.
Here is the basic ordering to initiate and complete the lock protocol when operating on an inode in UFS.
1) Acquire i_rwlock (from the vnode layer in most cases). 2) Begin the UFS lock protocol by calling ufs_lockfs_begin(). 3) Open UFS logging transactions if necessary now. 4) Acquire inode and quota locks (vfs_dqrwlock, i_contents, i_tlock, ...). 5) [work on inode] 6) Drop inode and quota locks (i_tlock, i_contents, vfs_dqrwlock, ...). 7) Close logging transactions. 8) End the UFS lock protocol by calling ufs_lockfs_end(). 9) Release i_rwlock.
When working with directories, you need to make one minor change. i_rwlock is acquired after the logging transaction is initialized, and i_rwlock is released before the transaction is ended. Here are the steps.
1) Begin the UFS lock protocol by calling ufs_lockfs_begin(). 2) Open UFS logging transactions if necessary now. 3) Acquire i_rwlock. 4) Acquire inode and quota locks (vfs_dqrwlock, i_contents, i_tlock, ...). 5) [work on inode] 6) Drop inode and quota locks (i_tlock, i_contents, vfs_dqrwlock, ...). 7) Release i_rwlock. 8) Close logging transactions. 9) End the UFS lock protocol by calling ufs_lockfs_end().