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-File management in the Linux kernel
------------------------------------
-
-This document describes how locking for files (struct file)
-and file descriptor table (struct files) works.
-
-Up until 2.6.12, the file descriptor table has been protected
-with a lock (files->file_lock) and reference count (files->count).
-->file_lock protected accesses to all the file related fields
-of the table. ->count was used for sharing the file descriptor
-table between tasks cloned with CLONE_FILES flag. Typically
-this would be the case for posix threads. As with the common
-refcounting model in the kernel, the last task doing
-a put_files_struct() frees the file descriptor (fd) table.
-The files (struct file) themselves are protected using
-reference count (->f_count).
-
-In the new lock-free model of file descriptor management,
-the reference counting is similar, but the locking is
-based on RCU. The file descriptor table contains multiple
-elements - the fd sets (open_fds and close_on_exec, the
-array of file pointers, the sizes of the sets and the array
-etc.). In order for the updates to appear atomic to
-a lock-free reader, all the elements of the file descriptor
-table are in a separate structure - struct fdtable.
-files_struct contains a pointer to struct fdtable through
-which the actual fd table is accessed. Initially the
-fdtable is embedded in files_struct itself. On a subsequent
-expansion of fdtable, a new fdtable structure is allocated
-and files->fdtab points to the new structure. The fdtable
-structure is freed with RCU and lock-free readers either
-see the old fdtable or the new fdtable making the update
-appear atomic. Here are the locking rules for
-the fdtable structure -
-
-1. All references to the fdtable must be done through
-   the files_fdtable() macro :
-
-	struct fdtable *fdt;
-
-	rcu_read_lock();
-
-	fdt = files_fdtable(files);
-	....
-	if (n <= fdt->max_fds)
-		....
-	...
-	rcu_read_unlock();
-
-   files_fdtable() uses rcu_dereference() macro which takes care of
-   the memory barrier requirements for lock-free dereference.
-   The fdtable pointer must be read within the read-side
-   critical section.
-
-2. Reading of the fdtable as described above must be protected
-   by rcu_read_lock()/rcu_read_unlock().
-
-3. For any update to the fd table, files->file_lock must
-   be held.
-
-4. To look up the file structure given an fd, a reader
-   must use either fcheck() or fcheck_files() APIs. These
-   take care of barrier requirements due to lock-free lookup.
-   An example :
-
-	struct file *file;
-
-	rcu_read_lock();
-	file = fcheck(fd);
-	if (file) {
-		...
-	}
-	....
-	rcu_read_unlock();
-
-5. Handling of the file structures is special. Since the look-up
-   of the fd (fget()/fget_light()) are lock-free, it is possible
-   that look-up may race with the last put() operation on the
-   file structure. This is avoided using atomic_long_inc_not_zero()
-   on ->f_count :
-
-	rcu_read_lock();
-	file = fcheck_files(files, fd);
-	if (file) {
-		if (atomic_long_inc_not_zero(&file->f_count))
-			*fput_needed = 1;
-		else
-		/* Didn't get the reference, someone's freed */
-			file = NULL;
-	}
-	rcu_read_unlock();
-	....
-	return file;
-
-   atomic_long_inc_not_zero() detects if refcounts is already zero or
-   goes to zero during increment. If it does, we fail
-   fget()/fget_light().
-
-6. Since both fdtable and file structures can be looked up
-   lock-free, they must be installed using rcu_assign_pointer()
-   API. If they are looked up lock-free, rcu_dereference()
-   must be used. However it is advisable to use files_fdtable()
-   and fcheck()/fcheck_files() which take care of these issues.
-
-7. While updating, the fdtable pointer must be looked up while
-   holding files->file_lock. If ->file_lock is dropped, then
-   another thread expand the files thereby creating a new
-   fdtable and making the earlier fdtable pointer stale.
-   For example :
-
-	spin_lock(&files->file_lock);
-	fd = locate_fd(files, file, start);
-	if (fd >= 0) {
-		/* locate_fd() may have expanded fdtable, load the ptr */
-		fdt = files_fdtable(files);
-		__set_open_fd(fd, fdt);
-		__clear_close_on_exec(fd, fdt);
-		spin_unlock(&files->file_lock);
-	.....
-
-   Since locate_fd() can drop ->file_lock (and reacquire ->file_lock),
-   the fdtable pointer (fdt) must be loaded after locate_fd().
-