Level 3

The level 3 lock prevents all processes except the lock owner from reading or writing to the disk. Read operations are blocked, and write operations and new file mappings are either blocked or failed depending on the permissions set in the level 1 lock. This is the most restrictive lock, not only because it prevents all other processes from accessing the disk but also because the lock owner is limited in what it can do.

Because read operations are blocked in the level 3 lock, an application must not execute any user interface or screen update functions, spawn applications, load dynamic-link libraries (DLLs), or yield to avoid deadlock. For example, if an application yields to a process with a discardable segment that the system has discarded, the system cannot reload the discarded segment because the process cannot read from the disk. This situation results in deadlock. Whenever a process obtains a level 3 lock, it should keep the lock for as short a period as possible to avoid severely degrading system performance. The purpose of the level 3 lock is to write changes to disk, so processes should only call disk I/O functions inside the lock.

After a level 3 lock is obtained, the file system takes several steps to allow a process to write directly to the disk. First, the system flushes all file system buffers and caches. Next, it puts the cache into write-through mode so that changes will be written to disk immediately. Finally, all open files are closed at the file system driver (FSD) level, which is invisible to all processes. While an application is in a level 3 lock, the system does not permit the swap file to grow or shrink, but it can still be read from or written to.

If an application has obtained a lock on a child volume, the system does not automatically flush the file system cache after a write operation to the parent volume. To ensure that the cache is flushed when Open or Create File (Interrupt 21h Function 716Ch) is used, the application should specify the OPEN_FLAGS_NO_BUFFERING (0100h) value. If the file has not been opened with OPEN_FLAGS_NO_BUFFERING or Absolute Disk Write (Interrupt 26h) has been used to write to the parent volume, the application should call Reset Drive (Interrupt 21h Function 710Dh) to flush the cache.

Before releasing the level 3 lock, the process is responsible for putting the file system into a state consistent with what existed before the lock was obtained. The process must be careful to correctly update all file system data, such as the FAT or directory entries. If a file was opened, it must not be deleted, renamed, or moved to a different volume. Otherwise, the system can become very unstable. An application should use Enumerate Open Files (Interrupt 21h Function 440Dh Minor Code 6Dh) to obtain a list of all open files on the volume.

When the level 3 lock is released, the system unblocks all pending read operations, reopens closed files on demand, and puts the cache back into write-behind mode. The lock owner returns to a level 2 lock.