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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().