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NAME

     mtx_pool, mtx_pool_alloc, mtx_pool_find, mtx_pool_lock,
     mtx_pool_lock_spin, mtx_pool_unlock, mtx_pool_unlock_spin,
     mtx_pool_create, mtx_pool_destroy - mutex pool routines

SYNOPSIS

     #include <sys/param.h>
     #include <sys/lock.h>
     #include <sys/mutex.h>

     struct mtx *
     mtx_pool_alloc(struct mtx_pool *pool);

     struct mtx *
     mtx_pool_find(struct mtx_pool *pool, void *ptr);

     void
     mtx_pool_lock(struct mtx_pool *pool, void *ptr);

     void
     mtx_pool_lock_spin(struct mtx_pool *pool, void *ptr);

     void
     mtx_pool_unlock(struct mtx_pool *pool, void *ptr);

     void
     mtx_pool_unlock_spin(struct mtx_pool *pool, void *ptr);

     struct mtx_pool *
     mtx_pool_create(const char *mtx_name, int pool_size, int opts);

     void
     mtx_pool_destroy(struct mtx_pool **poolp);

DESCRIPTION

     Mutex pools are designed to be used as short term leaf mutexes; i.e., the
     last mutex one might acquire before calling mtx_sleep(9).  They operate
     using a shared pool of mutexes.  A mutex may be chosen from the pool
     based on a supplied pointer, which may or may not point to anything
     valid, or the caller may allocate an arbitrary shared mutex from the pool
     and save the returned mutex pointer for later use.

     The shared mutexes in the mtxpool_sleep mutex pool, which is created by
     default, are standard, non-recursive, blockable mutexes, and should only
     be used in appropriate situations.  The mutexes in the
     mtxpool_lockbuilder mutex pool are similar, except that they are
     initialized with the MTX_NOWITNESS flag so that they may be used to build
     higher-level locks.  Other mutex pools may be created that contain
     mutexes with different properties, such as spin mutexes.

     The caller can lock and unlock mutexes returned by the pool routines, but
     since the mutexes are shared, the caller should not attempt to destroy
     them or modify their characteristics.  While pool mutexes are normally
     leaf mutexes (meaning that one cannot depend on any ordering guarantees
     after obtaining one), one can still obtain other mutexes under carefully
     controlled circumstances.  Specifically, if one has a private mutex (one
     that was allocated and initialized by the caller), one can obtain it
     after obtaining a pool mutex if ordering issues are carefully accounted
     for.  In these cases the private mutex winds up being the true leaf
     mutex.

     Pool mutexes have the following advantages:

           1.   No structural overhead; i.e., they can be associated with a
                structure without adding bloat to it.
           2.   Mutexes can be obtained for invalid pointers, which is useful
                when one uses mutexes to interlock destructor operations.
           3.   No initialization or destruction overhead.
           4.   Can be used with mtx_sleep(9).

     And the following disadvantages:

           1.   Should generally only be used as leaf mutexes.
           2.   Pool/pool dependency ordering cannot be guaranteed.
           3.   Possible L1 cache mastership contention between CPUs.

     mtx_pool_alloc() obtains a shared mutex from the specified pool.  This
     routine uses a simple rover to choose one of the shared mutexes managed
     by the mtx_pool subsystem.

     mtx_pool_find() returns the shared mutex associated with the specified
     address.  This routine will create a hash out of the pointer passed into
     it and will choose a shared mutex from the specified pool based on that
     hash.  The pointer does not need to point to anything real.

     mtx_pool_lock(), mtx_pool_lock_spin(), mtx_pool_unlock(), and
     mtx_pool_unlock_spin() lock and unlock the shared mutex from the
     specified pool associated with the specified address; they are a
     combination of mtx_pool_find() and mtx_lock(9), mtx_lock_spin(9),
     mtx_unlock(9), and mtx_unlock_spin(9), respectively.  Since these
     routines must first find the mutex to operate on, they are not as fast as
     directly using the mutex pointer returned by a previous invocation of
     mtx_pool_find() or mtx_pool_alloc().

     mtx_pool_create() allocates and initializes a new mutex pool of the
     specified size.  The pool size must be a power of two.  The opts argument
     is passed to mtx_init(9) to set the options for each mutex in the pool.

     mtx_pool_destroy() calls mtx_destroy(9) on each mutex in the specified
     pool, deallocates the memory associated with the pool, and assigns NULL
     to the pool pointer.

SEE ALSO

     locking(9) mutex(9)

HISTORY

     These routines first appeared in FreeBSD 5.0.