illumos: manual page: mutex

MUTEX(9F) Kernel Functions for Drivers MUTEX(9F)

NAME

mutex, mutex_enter, mutex_exit, mutex_init, mutex_destroy, mutex_owned,
mutex_tryenter - mutual exclusion lock routines

SYNOPSIS

#include <sys/ksynch.h>

void mutex_init(kmutex_t *mp, char *name, kmutex_type_t type,
void *arg);

void mutex_destroy(kmutex_t *mp);

void mutex_enter(kmutex_t *mp);

void mutex_exit(kmutex_t *mp);

int mutex_owned(kmutex_t *mp);

int mutex_tryenter(kmutex_t *mp);

INTERFACE LEVEL

illumos DDI specific (illumos DDI).

PARAMETERS

mp
Pointer to a kernel mutex lock (kmutex_t).

name
Descriptive string. This is obsolete and should be NULL.
(Non-NULL strings are legal, but they are a waste of kernel
memory.)

type
Type of mutex lock.

arg
Type-specific argument for initialization routine.

DESCRIPTION

A mutex enforces a policy of mutual exclusion. Only one thread at a time
may hold a particular mutex. Threads trying to lock a held mutex will
block until the mutex is unlocked.

Mutexes are strictly bracketing and may not be recursively locked,
meaning that mutexes should be exited in the opposite order they were
entered, and cannot be reentered before exiting.

mutex_init() initializes a mutex. It is an error to initialize a mutex
more than once. The type argument should be set to MUTEX_DRIVER.

arg provides type-specific information for a given variant type of mutex.
When mutex_init() is called for driver mutexes, if the mutex is used by
the interrupt handler, the arg should be the interrupt priority returned
from ddi_intr_get_pri(9F) or ddi_intr_get_softint_pri(9F). Note that arg
should be the value of the interrupt priority cast by calling the
DDI_INTR_PRI macro. If the mutex is never used inside an interrupt
handler, the argument should be NULL.

mutex_enter() is used to acquire a mutex. If the mutex is already held,
then the caller blocks. After returning, the calling thread is the owner
of the mutex. If the mutex is already held by the calling thread, a panic
ensues.

mutex_owned() should only be used in ASSERT() and may be enforced by not
being defined unless the preprocessor symbol DEBUG is defined. Its return
value is non-zero if the current thread (or, if that cannot be
determined, at least some thread) holds the mutex pointed to by mp.

mutex_tryenter() is very similar to mutex_enter() except that it doesn't
block when the mutex is already held. mutex_tryenter() returns non-zero
when it acquired the mutex and 0 when the mutex is already held.

mutex_exit() releases a mutex and will unblock another thread if any are
blocked on the mutex.

mutex_destroy() releases any resources that might have been allocated by
mutex_init(). mutex_destroy() must be called before freeing the memory
containing the mutex, and should be called with the mutex unheld (not
owned by any thread). The caller must be sure that no other thread
attempts to use the mutex.

RETURN VALUES

mutex_tryenter() returns a non-zero value on success and zero on failure.

mutex_owned() returns a non-zero value if the calling thread currently
holds the mutex pointed to by mp, or when that cannot be determined, if
any thread holds the mutex. Otherwise mutex_owned() returns zero.

CONTEXT

These functions can be called from user, kernel, or high-level interrupt
context, except for mutex_init() and mutex_destroy(), which can be called
from user or kernel context only.

EXAMPLES

Example 1: Initializing a Mutex

A driver might do this to initialize a mutex that is part of its unit
structure and used in its interrupt routine:

ddi_intr_get_pri(hdlp, &pri);
mutex_init(&un->un_lock, NULL, MUTEX_DRIVER, DDI_INTR_PRI(pri));
ddi_intr_add_handler(hdlp, xxintr, (caddr_t)un, NULL);

Example 2: Calling a Routine with a Lock

A routine that expects to be called with a certain lock held might have
the following ASSERT:

xxstart(struct xxunit *un)
{
ASSERT(mutex_owned(&un->un_lock));
...

NOTES

Compiling with _LOCKTEST or _MPSTATS defined has no effect. To gather
lock statistics, see lockstat(8).

The address of a kmutex_t lock must be aligned on an 8-byte boundary for
64-bit kernels, or a 4-byte boundary for 32-bit kernels. Violation of
this requirement will result in undefined behavior, including, but not
limited to, failure of mutual exclusion or a system panic.

To write scalable, responsive drivers that do not hang, panic or deadlock
the system, follow these guidelines:
Never return from a driver entry point with a mutex held.
Never hold a mutex when calling a service that may block, for example
kmem_alloc(9F) with KM_SLEEP or delay(9F).
Always acquire mutexes in a consistent order. If a critical section
acquires mutex A followed by B, and elsewhere in the driver mutex B is
acquired before A, the driver can deadlock with one thread holding A
and waiting for B and another thread holding B while waiting for A.
Always use a mutex to enforce exclusive access to data, not instruction
paths.
Acquiring a lock in user context that is also acquired in interrupt
context means that, as long as that lock is held, the driver instance
holding the lock is subject to all the rules and limitations of
interrupt context.
In most cases, a mutex can and should be acquired and released within
the same function.
Liberal use of debugging aids like ASSERT(mutex_owned(&mutex)) can help
find callers of a function which should be holding a mutex but are not.
This means you need to test your driver compiled with DEBUG.
Do not use a mutex to set driver state. However, you should use a mutex
to protect driver state data.
Use per-instance and automatic data where possible to reduce the amount
of shared data. Per-instance data can be protected by a per-instance
lock to improve scalability and reduce contention with multiple
hardware instances.
Avoid global data and global mutexes whenever possible.

May 21, 2008 MUTEX(9F)