[PATCH] mutex subsystem, documentation

   <title>Two Main Types of Kernel Locks: Spinlocks and Semaphores</title>

   <para>

     There are two main types of kernel locks.  The fundamental type

     There are three main types of kernel locks.  The fundamental type

     is the spinlock 

     (<filename class="headerfile">include/asm/spinlock.h</filename>),

     which is a very simple single-holder lock: if you can't get the 

     very small and fast, and can be used anywhere.

   </para>

   <para>

     The second type is a semaphore

     The second type is a mutex

     (<filename class="headerfile">include/linux/mutex.h</filename>): it

     is like a spinlock, but you may block holding a mutex.

     If you can't lock a mutex, your task will suspend itself, and be woken

     up when the mutex is released.  This means the CPU can do something

     else while you are waiting.  There are many cases when you simply

     can't sleep (see <xref linkend="sleeping-things"/>), and so have to

     use a spinlock instead.

   </para>

   <para>

     The third type is a semaphore

     (<filename class="headerfile">include/asm/semaphore.h</filename>): it

     can have more than one holder at any time (the number decided at

     initialization time), although it is most commonly used as a

     single-holder lock (a mutex).  If you can't get a semaphore,

     your task will put itself on the queue, and be woken up when the

     semaphore is released.  This means the CPU will do something

     else while you are waiting, but there are many cases when you

     simply can't sleep (see <xref linkend="sleeping-things"/>), and so

     have to use a spinlock instead.

     single-holder lock (a mutex).  If you can't get a semaphore, your

     task will be suspended and later on woken up - just like for mutexes.

   </para>

   <para>

     Neither type of lock is recursive: see

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 irq contexts

   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);