July 19, 2008 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

```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 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

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 attempting 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 handlers.

    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 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.

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 substantially 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 substantially 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)

### HISTORY

The mutex primitives first appeared in NetBSD5.0.