FreeBSD/Linux Kernel Cross Reference
sys/Documentation/volatile-considered-harmful.txt

     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.

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