MUTEX(9)               NetBSD Kernel Developer's Manual               MUTEX(9)

NAME
     mutex, mutex_init, mutex_destroy, mutex_enter, mutex_exit, mutex_owned,
     mutex_spin_enter, mutex_spin_exit, mutex_tryenter -- mutual exclusion
     primitives

SYNOPSIS
     #include <sys/mutex.h>

     void
     mutex_init(kmutex_t *mtx, kmutex_type_t type, int ipl);

     void
     mutex_destroy(kmutex_t *mtx);

     void
     mutex_enter(kmutex_t *mtx);

     void
     mutex_exit(kmutex_t *mtx);

     int
     mutex_owned(kmutex_t *mtx);

     void
     mutex_spin_enter(kmutex_t *mtx);

     void
     mutex_spin_exit(kmutex_t *mtx);

     int
     mutex_tryenter(kmutex_t *mtx);

     options DIAGNOSTIC
     options LOCKDEBUG

DESCRIPTION
     Mutexes are used in the kernel to implement mutual exclusion among LWPs
     (lightweight processes) and interrupt handlers.

     The kmutex_t type provides storage for the mutex object.  This should be
     treated as an opaque object and not examined directly by consumers.

     Mutexes replace the spl(9) system traditionally used to provide synchro-
     nization between interrupt handlers and LWPs.

OPTIONS
     options DIAGNOSTIC

           Kernels compiled with the DIAGNOSTIC option perform basic sanity
           checks on mutex operations.

     options LOCKDEBUG

           Kernels compiled with the LOCKDEBUG option perform potentially CPU
           intensive sanity checks on mutex operations.

FUNCTIONS
     mutex_init(mtx, type, ipl)

           Dynamically initialize a mutex for use.

           No other operations can be performed on a mutex until it has been
           initialized.  Once initialized, all types of mutex are manipulated
           using the same interface.  Note that mutex_init() may block in
           order to allocate memory.

           The type argument must be given as MUTEX_DEFAULT.  Other constants
           are defined but are for low-level system use and are not an
           endorsed, stable part of the interface.

           The type of mutex returned depends on the ipl argument:

           IPL_NONE, or one of the IPL_SOFT* constants

                 An adaptive mutex will be returned.  Adaptive mutexes provide
                 mutual exclusion between LWPs, and between LWPs and soft
                 interrupt handlers.

                 Adaptive mutexes cannot be acquired from a hardware interrupt
                 handler.  An LWP may either sleep or busy-wait when attempt-
                 ing to acquire an adaptive mutex that is already held.

           IPL_VM, IPL_SCHED, IPL_HIGH

                 A spin mutex will be returned.  Spin mutexes provide mutual
                 exclusion between LWPs, and between LWPs and interrupt han-
                 dlers.

                 The ipl argument is used to pass a system interrupt priority
                 level (IPL) that will block all interrupt handlers that may
                 try to acquire the mutex.

                 LWPs that own spin mutexes may not sleep, and therefore must
                 not try to acquire adaptive mutexes or other sleep locks.

                 A processor will always busy-wait when attempting to acquire
                 a spin mutex that is already held.

           See spl(9) for further information on interrupt priority levels
           (IPLs).

     mutex_destroy(mtx)

           Release resources used by a mutex.  The mutex may not be used after
           it has been destroyed.  mutex_destroy() may block in order to free
           memory.

     mutex_enter(mtx)

           Acquire a mutex.  If the mutex is already held, the caller will
           block and not return until the mutex is acquired.

           Mutexes and other types of locks must always be acquired in a con-
           sistent order with respect to each other.  Otherwise, the potential
           for system deadlock exists.

           Adaptive mutexes and other types of lock that can sleep may not be
           acquired while a spin mutex is held by the caller.

     mutex_exit(mtx)

           Release a mutex.  The mutex must have been previously acquired by
           the caller.  Mutexes may be released out of order as needed.

     mutex_owned(mtx)

           For adaptive mutexes, return non-zero if the current LWP holds the
           mutex.  For spin mutexes, return non-zero if the mutex is held,
           potentially by the current processor.  Otherwise, return zero.

           mutex_owned() is provided for making diagnostic checks to verify
           that a lock is held.  For example:

                   KASSERT(mutex_owned(&driver_lock));

           It should not be used to make locking decisions at run time, or to
           verify that a lock is not held.

     mutex_spin_enter(mtx)

           Equivalent to mutex_enter(), but may only be used when it is known
           that mtx is a spin mutex.  On some architectures, this can substan-
           tially reduce the cost of acquring a spin mutex.

     mutex_spin_exit(mtx)

           Equivalent to mutex_exit(), but may only be used when it is known
           that mtx is a spin mutex.  On some architectures, this can substan-
           tially reduce the cost of releasing a spin mutex.

     mutex_tryenter(mtx)

           Try to acquire a mutex, but do not block if the mutex is already
           held.  Returns non-zero if the mutex was acquired, or zero if the
           mutex was already held.

           mutex_tryenter() can be used as an optimization when acquiring
           locks in the the wrong order.  For example, in a setting where the
           convention is that first_lock must be acquired before second_lock,
           the following can be used to optimistically lock in reverse order:

                   /* We hold second_lock, but not first_lock. */
                   KASSERT(mutex_owned(&second_lock));

                   if (!mutex_tryenter(&first_lock)) {
                           /* Failed to get it - lock in the correct order. */
                           mutex_exit(&second_lock);
                           mutex_enter(&first_lock);
                           mutex_enter(&second_lock);

                           /*
                            * We may need to recheck any conditions the code
                            * path depends on, as we released second_lock
                            * briefly.
                            */
                   }

CODE REFERENCES
     This section describes places within the NetBSD source tree where code
     implementing mutexes can be found.  All pathnames are relative to
     /usr/src.

     The core of the mutex implementation is in sys/kern/kern_mutex.c.

     The header file sys/sys/mutex.h describes the public interface, and
     interfaces that machine-dependent code must provide to support mutexes.

SEE ALSO
     atomic_ops(3), membar_ops(3), condvar(9), rwlock(9), spl(9)

     Jim Mauro and Richard McDougall, Solaris Internals: Core Kernel
     Architecture,, Prentice Hall, 2001, ISBN 0-13-022496-0.

HISTORY
     The mutex primitives first appeared in NetBSD 5.0.

NetBSD 5.0_STABLE                July 19, 2008               NetBSD 5.0_STABLE
