Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

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

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

     #include <sys/mutex.h>

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

     mutex_destroy(kmutex_t *mtx);

     mutex_enter(kmutex_t *mtx);

     mutex_exit(kmutex_t *mtx);

     mutex_ownable(kmutex_t *mtx);

     mutex_owned(kmutex_t *mtx);

     mutex_spin_enter(kmutex_t *mtx);

     mutex_spin_exit(kmutex_t *mtx);

     mutex_tryenter(kmutex_t *mtx);

     options DIAGNOSTIC
     options LOCKDEBUG

     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
     synchronization between interrupt handlers and LWPs.

     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.

     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
                 attempting to acquire an adaptive mutex that is already held.


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

                 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.

                 Note: Releasing a spin mutex may not lower the IPL to what it
                 was when entered.  If other spin mutexes are held, the IPL
                 will not be lowered until the last one is released.

                 This is usually not a problem because spin mutexes should
                 held only for very short durations anyway, so blocking
                 higher-priority interrupts a little longer doesn't hurt much.
                 But it interferes with writing assertions that the IPL is no
                 higher than a specified level.

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


           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


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

           All loads and stores after mutex_enter() will not be reordered
           before it or served from a prior cache, and hence will happen after
           any prior mutex_exit() to release the mutex even on another CPU or
           in an interrupt.  Thus, there is a global total ordering on all
           loads and stores under the same mutex.

           Mutexes and other types of locks must always be acquired in a
           consistent 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.

           When acquiring a spin mutex, the IPL of the current CPU will be
           raised to the level set in mutex_init() if it is not already equal
           or higher.


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

           All loads and stores before mutex_exit() will not be reordered
           after it or delayed in a write buffer, and hence will
           happen(before) any subsequent mutex_enter() to acquire the mutex
           even on another CPU or in an interrupt.  Thus, there is a global
           total ordering on all loads and stores under the same mutex.


           When compiled with LOCKDEBUG (see options(4)), ensure that the
           current process can successfully acquire mtx.  If mtx is already
           owned by the current process, the system will panic with a "locking
           against myself" error.

           This function is needed because mutex_owned() does not
           differentiate if a spin mutex is owned by the current process vs
           owned by another process.  mutex_ownable() is reasonably heavy-
           weight, and should only be used with KDASSERT(9).


           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:


           It should not be used to make locking decisions at run time.  For
           spin mutexes, it must not be used to verify that a lock is not


           Equivalent to mutex_enter(), but may only be used when it is known
           that mtx is a spin mutex.  Implies the same memory ordering as
           mutex_enter().  On some architectures, this can substantially
           reduce the cost of acquiring a spin mutex.


           Equivalent to mutex_exit(), but may only be used when it is known
           that mtx is a spin mutex.  Implies the same memory ordering as
           mutex_exit().  On some architectures, this can substantially reduce
           the cost of releasing a spin mutex.


           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 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. */

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

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

     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.

     atomic_ops(3), membar_ops(3), lockstat(8), condvar(9), kpreempt(9),
     rwlock(9), spl(9)

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

     The mutex primitives first appeared in NetBSD 5.0.

NetBSD 9.99                    December 8, 2017                    NetBSD 9.99