Tribblix: manual page: mutex.9f

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 This is obsolete and should be NULL.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)