Documentation/filesystems/files.txt

   1 File management in the Linux kernel
   2 -----------------------------------
   3
   4 This document describes how locking for files (struct file)
   5 and file descriptor table (struct files) works.
   6
   7 Up until 2.6.12, the file descriptor table has been protected
   8 with a lock (files->file_lock) and reference count (files->count).
   9 ->file_lock protected accesses to all the file related fields
  10 of the table. ->count was used for sharing the file descriptor
  11 table between tasks cloned with CLONE_FILES flag. Typically
  12 this would be the case for posix threads. As with the common
  13 refcounting model in the kernel, the last task doing
  14 a put_files_struct() frees the file descriptor (fd) table.
  15 The files (struct file) themselves are protected using
  16 reference count (->f_count).
  17
  18 In the new lock-free model of file descriptor management,
  19 the reference counting is similar, but the locking is
  20 based on RCU. The file descriptor table contains multiple
  21 elements - the fd sets (open_fds and close_on_exec, the
  22 array of file pointers, the sizes of the sets and the array
  23 etc.). In order for the updates to appear atomic to
  24 a lock-free reader, all the elements of the file descriptor
  25 table are in a separate structure - struct fdtable.
  26 files_struct contains a pointer to struct fdtable through
  27 which the actual fd table is accessed. Initially the
  28 fdtable is embedded in files_struct itself. On a subsequent
  29 expansion of fdtable, a new fdtable structure is allocated
  30 and files->fdtab points to the new structure. The fdtable
  31 structure is freed with RCU and lock-free readers either
  32 see the old fdtable or the new fdtable making the update
  33 appear atomic. Here are the locking rules for
  34 the fdtable structure -
  35
  36 1. All references to the fdtable must be done through
  37    the files_fdtable() macro :
  38
  39         struct fdtable *fdt;
  40
  41         rcu_read_lock();
  42
  43         fdt = files_fdtable(files);
  44         ....
  45         if (n <= fdt->max_fds)
  46                 ....
  47         ...
  48         rcu_read_unlock();
  49
  50    files_fdtable() uses rcu_dereference() macro which takes care of
  51    the memory barrier requirements for lock-free dereference.
  52    The fdtable pointer must be read within the read-side
  53    critical section.
  54
  55 2. Reading of the fdtable as described above must be protected
  56    by rcu_read_lock()/rcu_read_unlock().
  57
  58 3. For any update to the fd table, files->file_lock must
  59    be held.
  60
  61 4. To look up the file structure given an fd, a reader
  62    must use either fcheck() or fcheck_files() APIs. These
  63    take care of barrier requirements due to lock-free lookup.
  64    An example :
  65
  66         struct file *file;
  67
  68         rcu_read_lock();
  69         file = fcheck(fd);
  70         if (file) {
  71                 ...
  72         }
  73         ....
  74         rcu_read_unlock();
  75
  76 5. Handling of the file structures is special. Since the look-up
  77    of the fd (fget()/fget_light()) are lock-free, it is possible
  78    that look-up may race with the last put() operation on the
  79    file structure. This is avoided using atomic_long_inc_not_zero()
  80    on ->f_count :
  81
  82         rcu_read_lock();
  83         file = fcheck_files(files, fd);
  84         if (file) {
  85                 if (atomic_long_inc_not_zero(&file->f_count))
  86                         *fput_needed = 1;
  87                 else
  88                 /* Didn't get the reference, someone's freed */
  89                         file = NULL;
  90         }
  91         rcu_read_unlock();
  92         ....
  93         return file;
  94
  95    atomic_long_inc_not_zero() detects if refcounts is already zero or
  96    goes to zero during increment. If it does, we fail
  97    fget()/fget_light().
  98
  99 6. Since both fdtable and file structures can be looked up
 100    lock-free, they must be installed using rcu_assign_pointer()
 101    API. If they are looked up lock-free, rcu_dereference()
 102    must be used. However it is advisable to use files_fdtable()
 103    and fcheck()/fcheck_files() which take care of these issues.
 104
 105 7. While updating, the fdtable pointer must be looked up while
 106    holding files->file_lock. If ->file_lock is dropped, then
 107    another thread expand the files thereby creating a new
 108    fdtable and making the earlier fdtable pointer stale.
 109    For example :
 110
 111         spin_lock(&files->file_lock);
 112         fd = locate_fd(files, file, start);
 113         if (fd >= 0) {
 114                 /* locate_fd() may have expanded fdtable, load the ptr */
 115                 fdt = files_fdtable(files);
 116                 __set_open_fd(fd, fdt);
 117                 __clear_close_on_exec(fd, fdt);
 118                 spin_unlock(&files->file_lock);
 119         .....
 120
 121    Since locate_fd() can drop ->file_lock (and reacquire ->file_lock),
 122    the fdtable pointer (fdt) must be loaded after locate_fd().
 123