Generic Mutex Subsystem
  
  started by Ingo Molnar <mingo@redhat.com>
  
    "Why on earth do we need a new mutex subsystem, and what's wrong
     with semaphores?"
  
  firstly, there's nothing wrong with semaphores. But if the simpler
  mutex semantics are sufficient for your code, then there are a couple
  of advantages of mutexes:

   - 'struct mutex' is smaller on most architectures: E.g. on x86,

     'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes.
     A smaller structure size means less RAM footprint, and better
     CPU-cache utilization.
  
   - tighter code. On x86 i get the following .text sizes when
     switching all mutex-alike semaphores in the kernel to the mutex
     subsystem:
  
          text    data     bss     dec     hex filename
       3280380  868188  396860 4545428  455b94 vmlinux-semaphore
       3255329  865296  396732 4517357  44eded vmlinux-mutex
  
     that's 25051 bytes of code saved, or a 0.76% win - off the hottest
     codepaths of the kernel. (The .data savings are 2892 bytes, or 0.33%)
     Smaller code means better icache footprint, which is one of the
     major optimization goals in the Linux kernel currently.
  
   - the mutex subsystem is slightly faster and has better scalability for
     contended workloads. On an 8-way x86 system, running a mutex-based
     kernel and testing creat+unlink+close (of separate, per-task files)
     in /tmp with 16 parallel tasks, the average number of ops/sec is:
  
      Semaphores:                        Mutexes:
  
      $ ./test-mutex V 16 10             $ ./test-mutex V 16 10
      8 CPUs, running 16 tasks.          8 CPUs, running 16 tasks.
      checking VFS performance.          checking VFS performance.
      avg loops/sec:      34713          avg loops/sec:      84153
      CPU utilization:    63%            CPU utilization:    22%
  
     i.e. in this workload, the mutex based kernel was 2.4 times faster
     than the semaphore based kernel, _and_ it also had 2.8 times less CPU
     utilization. (In terms of 'ops per CPU cycle', the semaphore kernel
     performed 551 ops/sec per 1% of CPU time used, while the mutex kernel
     performed 3825 ops/sec per 1% of CPU time used - it was 6.9 times
     more efficient.)
  
     the scalability difference is visible even on a 2-way P4 HT box:
  
      Semaphores:                        Mutexes:
  
      $ ./test-mutex V 16 10             $ ./test-mutex V 16 10
      4 CPUs, running 16 tasks.          8 CPUs, running 16 tasks.
      checking VFS performance.          checking VFS performance.
      avg loops/sec:      127659         avg loops/sec:      181082
      CPU utilization:    100%           CPU utilization:    34%
  
     (the straight performance advantage of mutexes is 41%, the per-cycle
      efficiency of mutexes is 4.1 times better.)
  
   - there are no fastpath tradeoffs, the mutex fastpath is just as tight
     as the semaphore fastpath. On x86, the locking fastpath is 2
     instructions:
  
      c0377ccb <mutex_lock>:
      c0377ccb:       f0 ff 08                lock decl (%eax)

      c0377cce:       78 0e                   js     c0377cde <.text..lock.mutex>

      c0377cd0:       c3                      ret
  
     the unlocking fastpath is equally tight:
  
      c0377cd1 <mutex_unlock>:
      c0377cd1:       f0 ff 00                lock incl (%eax)

      c0377cd4:       7e 0f                   jle    c0377ce5 <.text..lock.mutex+0x7>

      c0377cd6:       c3                      ret
  
   - 'struct mutex' semantics are well-defined and are enforced if
     CONFIG_DEBUG_MUTEXES is turned on. Semaphores on the other hand have
     virtually no debugging code or instrumentation. The mutex subsystem
     checks and enforces the following rules:
  
     * - only one task can hold the mutex at a time
     * - only the owner can unlock the mutex
     * - multiple unlocks are not permitted
     * - recursive locking is not permitted
     * - a mutex object must be initialized via the API
     * - a mutex object must not be initialized via memset or copying
     * - task may not exit with mutex held
     * - memory areas where held locks reside must not be freed
     * - held mutexes must not be reinitialized

     * - mutexes may not be used in hardware or software interrupt
     *   contexts such as tasklets and timers

  
     furthermore, there are also convenience features in the debugging
     code:
  
     * - uses symbolic names of mutexes, whenever they are printed in debug output
     * - point-of-acquire tracking, symbolic lookup of function names
     * - list of all locks held in the system, printout of them
     * - owner tracking
     * - detects self-recursing locks and prints out all relevant info
     * - detects multi-task circular deadlocks and prints out all affected
     *   locks and tasks (and only those tasks)
  
  Disadvantages
  -------------
  
  The stricter mutex API means you cannot use mutexes the same way you
  can use semaphores: e.g. they cannot be used from an interrupt context,
  nor can they be unlocked from a different context that which acquired
  it. [ I'm not aware of any other (e.g. performance) disadvantages from
  using mutexes at the moment, please let me know if you find any. ]
  
  Implementation of mutexes
  -------------------------
  
  'struct mutex' is the new mutex type, defined in include/linux/mutex.h
  and implemented in kernel/mutex.c. It is a counter-based mutex with a
  spinlock and a wait-list. The counter has 3 states: 1 for "unlocked",
  0 for "locked" and negative numbers (usually -1) for "locked, potential
  waiters queued".
  
  the APIs of 'struct mutex' have been streamlined:
  
   DEFINE_MUTEX(name);
  
   mutex_init(mutex);
  
   void mutex_lock(struct mutex *lock);
   int  mutex_lock_interruptible(struct mutex *lock);
   int  mutex_trylock(struct mutex *lock);
   void mutex_unlock(struct mutex *lock);
   int  mutex_is_locked(struct mutex *lock);

   void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
   int  mutex_lock_interruptible_nested(struct mutex *lock,
                                        unsigned int subclass);

   int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);