mutex(9F) Kernel Functions for Drivers mutex(9F)
mutex, mutex_enter, mutex_exit, mutex_init, mutex_destroy, mutex_owned, mutex_tryenter - mutual exclusion lock routines
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);
Solaris DDI specific (Solaris DDI).
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 This is obsolete and should be NULL.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 mem-
type Type of mutex lock.
arg Type-specific argument for initialization routine.
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 ddi_iblock_cookie returned from ddi_get_iblock_cookie(9F) or
ddi_get_soft_iblock_cookie(9F). Note that arg should be the value of the iblock cookie casted to (void *), not the address of the cookie.
The arguments passed to ddi_get_iblock_cookie(9F) and ddi_get_soft_iblock_cookie(9F), on the other hand, are the addresses of the cookie.
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
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.
mutex_tryenter() returns non-zero on success and zero on failure.
mutex_owned() returns non-zero if the calling thread currently holds the mutex pointed to by mp, or when that cannot be determined, if any
thread holds the mutex. mutex_owned() returns zero.
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.
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_get_iblock_cookie(dip, 0, &iblock);
mutex_init(&un->un_lock, NULL, MUTEX_DRIVER,
ddi_add_intr(dip, 0, NULL, &dev_cookie, xxintr,
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)
lockstat(1M), Intro(9F), condvar(9F), ddi_add_intr(9F), ddi_get_iblock_cookie(9F), ddi_get_soft_iblock_cookie(9F), rwlock(9F), sema-
Writing Device Drivers
Compiling with _LOCKTEST or _MPSTATS defined has no effect. To gather lock statistics, see lockstat(1M).
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.
SunOS 5.10 16 Sep 2003 mutex(9F)