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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 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 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.
Return non-zero if write lock is held by current lwp. Otherwise,
Returns non-zero if read lock is held by any lwp. Otherwise,
Returns non-zero if either read or write lock is held by any lwp.
Otherwise, return zero.
Functions rw_write_held(), rw_read_held(), and rw_lock_held() must
never be used to make locking decisions at run time: they are pro-
vided only for diagnostic purposes. They are also not atomic,
hence they should only be used to assert that lock is held. The
only exception is rw_write_held(), which can be also used safely to
assert that write lock is NOT currently held by current lwp.
RW locks are subject to high cache contention on multiprocessor systems,
and scale poorly when the write:read ratio is not strongly in favour of
readers. Ideally, RW locks should only be used in settings when the fol-
lowing three conditions are met:
· The data object(s) protected by the RW lock are read much more fre-
quently than written.
· The read-side hold time for the RW lock is long (in the order of
thousands of processor clock cycles).
· Strong synchronization semantics are required: there is no scope for
lockless, lazy or optimistic synchronization.
Generally speaking, it is better to organise code paths and/or data flows
such that fewer and weaker synchronization points are required to ensure
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.
membar_ops(3), lockstat(8), condvar(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 9.0 December 10, 2018 NetBSD 9.0