Why the "volatile" type class should not be used
  ------------------------------------------------
  
  C programmers have often taken volatile to mean that the variable could be
  changed outside of the current thread of execution; as a result, they are
  sometimes tempted to use it in kernel code when shared data structures are
  being used.  In other words, they have been known to treat volatile types
  as a sort of easy atomic variable, which they are not.  The use of volatile in
  kernel code is almost never correct; this document describes why.
  
  The key point to understand with regard to volatile is that its purpose is
  to suppress optimization, which is almost never what one really wants to
  do.  In the kernel, one must protect shared data structures against
  unwanted concurrent access, which is very much a different task.  The
  process of protecting against unwanted concurrency will also avoid almost
  all optimization-related problems in a more efficient way.
  
  Like volatile, the kernel primitives which make concurrent access to data
  safe (spinlocks, mutexes, memory barriers, etc.) are designed to prevent
  unwanted optimization.  If they are being used properly, there will be no
  need to use volatile as well.  If volatile is still necessary, there is
  almost certainly a bug in the code somewhere.  In properly-written kernel
  code, volatile can only serve to slow things down.
  
  Consider a typical block of kernel code:
  
      spin_lock(&the_lock);
      do_something_on(&shared_data);
      do_something_else_with(&shared_data);
      spin_unlock(&the_lock);
  
  If all the code follows the locking rules, the value of shared_data cannot
  change unexpectedly while the_lock is held.  Any other code which might
  want to play with that data will be waiting on the lock.  The spinlock
  primitives act as memory barriers - they are explicitly written to do so -
  meaning that data accesses will not be optimized across them.  So the
  compiler might think it knows what will be in shared_data, but the
  spin_lock() call, since it acts as a memory barrier, will force it to
  forget anything it knows.  There will be no optimization problems with
  accesses to that data.
  
  If shared_data were declared volatile, the locking would still be
  necessary.  But the compiler would also be prevented from optimizing access
  to shared_data _within_ the critical section, when we know that nobody else
  can be working with it.  While the lock is held, shared_data is not
  volatile.  When dealing with shared data, proper locking makes volatile
  unnecessary - and potentially harmful.
  
  The volatile storage class was originally meant for memory-mapped I/O
  registers.  Within the kernel, register accesses, too, should be protected
  by locks, but one also does not want the compiler "optimizing" register
  accesses within a critical section.  But, within the kernel, I/O memory
  accesses are always done through accessor functions; accessing I/O memory
  directly through pointers is frowned upon and does not work on all
  architectures.  Those accessors are written to prevent unwanted
  optimization, so, once again, volatile is unnecessary.
  
  Another situation where one might be tempted to use volatile is
  when the processor is busy-waiting on the value of a variable.  The right
  way to perform a busy wait is:
  
      while (my_variable != what_i_want)
          cpu_relax();
  
  The cpu_relax() call can lower CPU power consumption or yield to a

  hyperthreaded twin processor; it also happens to serve as a compiler
  barrier, so, once again, volatile is unnecessary.  Of course, busy-
  waiting is generally an anti-social act to begin with.

  
  There are still a few rare situations where volatile makes sense in the
  kernel:
  
    - The above-mentioned accessor functions might use volatile on
      architectures where direct I/O memory access does work.  Essentially,
      each accessor call becomes a little critical section on its own and
      ensures that the access happens as expected by the programmer.
  
    - Inline assembly code which changes memory, but which has no other
      visible side effects, risks being deleted by GCC.  Adding the volatile
      keyword to asm statements will prevent this removal.
  
    - The jiffies variable is special in that it can have a different value
      every time it is referenced, but it can be read without any special
      locking.  So jiffies can be volatile, but the addition of other
      variables of this type is strongly frowned upon.  Jiffies is considered
      to be a "stupid legacy" issue (Linus's words) in this regard; fixing it
      would be more trouble than it is worth.
  
    - Pointers to data structures in coherent memory which might be modified
      by I/O devices can, sometimes, legitimately be volatile.  A ring buffer
      used by a network adapter, where that adapter changes pointers to
      indicate which descriptors have been processed, is an example of this
      type of situation.
  
  For most code, none of the above justifications for volatile apply.  As a
  result, the use of volatile is likely to be seen as a bug and will bring
  additional scrutiny to the code.  Developers who are tempted to use
  volatile should take a step back and think about what they are truly trying
  to accomplish.
  
  Patches to remove volatile variables are generally welcome - as long as
  they come with a justification which shows that the concurrency issues have
  been properly thought through.
  
  
  NOTES
  -----
  
  [1] http://lwn.net/Articles/233481/
  [2] http://lwn.net/Articles/233482/
  
  CREDITS
  -------
  
  Original impetus and research by Randy Dunlap
  Written by Jonathan Corbet

  Improvements via comments from Satyam Sharma, Johannes Stezenbach, Jesper

  	Juhl, Heikki Orsila, H. Peter Anvin, Philipp Hahn, and Stefan
  	Richter.