bionic (9) mtx_pool.9freebsd.gz

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.