Synchronization

The provided file system requires external synchronization, that is, callers must ensure that only one thread can be running in the file system code at once. Your submission must adopt a finer-grained synchronization strategy that does not require external synchronization. To the extent possible, operations on independent entities should be independent, so that they do not need to wait on each other.

Operations on different cache blocks must be independent. In particular, when I/O is required on a particular block, operations on other blocks that do not require I/O should proceed without having to wait for the I/O to complete.

Multiple processes must be able to access a single file at once. Multiple reads of a single file must be able to complete without waiting for one another. When writing to a file does not extend the file, multiple processes should also be able to write a single file at once. A read of a file by one process when the file is being written by another process is allowed to show that none, all, or part of the write has completed. (However, after the write system call returns to its caller, all subsequent readers must see the change.) Similarly, when two processes simultaneously write to the same part of a file, their data may be interleaved.

On the other hand, extending a file and writing data into the new section must be atomic. Suppose processes A and B both have a given file open and both are positioned at end-of-file. If A reads and B writes the file at the same time, A may read all, part, or none of what B writes. However, A may not read data other than what B writes, e.g. if B's data is all nonzero bytes, A is not allowed to see any zeros.

Operations on different directories should take place concurrently. Operations on the same directory may wait for one another.

Keep in mind that only data shared by multiple threads needs to be synchronized. In the base file system, struct file and struct dir are accessed only by a single thread.