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MUTEX(9) Kernel Developer's Manual MUTEX(9)
NAME
mutex, mutex_init, mutex_destroy, mutex_enter, mutex_exit, mutex_ownable,
mutex_owned, mutex_spin_enter, mutex_spin_exit, mutex_tryenter - mutual
exclusion primitives
SYNOPSIS
#include <sys/mutex.h>
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_ownable(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 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
The following kernel options have effect on mutex operations:
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.
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
(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.
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.
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.
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.
mutex_ownable(mtx)
When compiled with LOCKDEBUG 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).
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. For
spin mutexes, it must not be used 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. Implies the same memory ordering
as mutex_enter(). On some architectures, this can substantially
reduce the cost of acquiring 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. Implies the same memory ordering as
mutex_exit(). 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 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
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), options(4), 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.
HISTORY
The mutex primitives first appeared in NetBSD 5.0. mutex_ownable() first
appeared in NetBSD 8.0.
NetBSD 10.99 December 8, 2017 NetBSD 10.99