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RWLOCK(9) NetBSD Kernel Developer's Manual RWLOCK(9)
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rw, rw_init, rw_destroy, rw_enter, rw_exit, rw_tryenter, rw_tryupgrade,
rw_downgrade, rw_read_held, rw_write_held, rw_lock_held -- reader /
writer lock primitives
rw_enter(krwlock_t *rw, const krw_t op);
rw_tryenter(krwlock_t *rw, const krw_t op);
Reader / writer locks (RW locks) are used in the kernel to synchronize
access to an object among LWPs (lightweight processes) and soft interrupt
In addition to the capabilities provided by mutexes, RW locks distinguish
between read (shared) and write (exclusive) access. RW locks are
intended to provide protection for kernel data or objects that are read
much more frequently than updated. For objects that are updated as fre-
quently as they are read, mutexes should be used to guarantee atomic
RW locks are in one of three distinct states at any given time:
Unlocked The lock is not held.
Read locked The lock holders intend to read the protected object. Mul-
tiple callers may hold a RW lock with ``read intent''
Write locked The lock holder intends to update the protected object.
Only one caller may hold a RW lock with ``write intent''.
The krwlock_t type provides storage for the RW lock object. This should
be treated as an opaque object and not examined directly by consumers.
Note that the these interfaces must not be used from a hardware interrupt
OPTIONS AND MACROS
Kernels compiled with the DIAGNOSTIC option perform basic sanity
checks on RW lock operations.
Kernels compiled with the LOCKDEBUG option perform potentially CPU
intensive sanity checks on RW lock operations.
Initialize a lock for use. No other operations can be performed on
the lock until it has been initialized.
Release resources used by a lock. The lock may not be used after
it has been destroyed.
If RW_READER is specified as the argument to op, acquire a read
lock. If the lock is write held, the caller will block and not
return until the hold is acquired. Callers must not recursively
acquire read locks.
If RW_WRITER is specified, acquire a write lock. If the lock is
already held, the caller will block and not return until the hold
RW locks and other types of locks must always be acquired in a con-
sistent order with respect to each other. Otherwise, the potential
for system deadlock exists.
Release a lock. The lock must have been previously acquired by the
Try to acquire a lock, but do not block if the lock is already
held. If the lock is acquired successfully, return non-zero. Oth-
erwise, return zero.
Valid arguments to op are RW_READER or RW_WRITER.
Try to upgrade a lock from one read hold to a write hold. If the
lock is upgraded successfully, returns non-zero. Otherwise,
Downgrade a lock from a write hold to a read hold.
Test the lock's condition and return non-zero if the lock is held
(potentially by the current LWP) and matches the specified condi-
tion. Otherwise, return zero.
These functions must never be used to make locking decisions at run
time: they are provided only for diagnostic purposes.
This section describes places within the NetBSD source tree where code
implementing RW locks can be found. All pathnames are relative to
The core of the RW lock implementation is in sys/kern/kern_rwlock.c.
The header file sys/sys/rwlock.h describes the public interface, and
interfaces that machine-dependent code must provide to support RW locks.
condvar(9), mb(9), mutex(9)
Jim Mauro and Richard McDougall, Solaris Internals: Core Kernel
Architecture, Prentice Hall, 2001, ISBN 0-13-022496-0.
The RW lock primitives first appeared in NetBSD 5.0.
NetBSD 5.0 December 4, 2007 NetBSD 5.0