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NetBSD 6.1.5 - man page for rw_downgrade (netbsd section 9)

RWLOCK(9)			  BSD Kernel Developer's Manual 			RWLOCK(9)

NAME
     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

SYNOPSIS
     #include <sys/rwlock.h>

     void
     rw_init(krwlock_t *rw);

     void
     rw_destroy(krwlock_t *rw);

     void
     rw_enter(krwlock_t *rw, const krw_t op);

     void
     rw_exit(krwlock_t *rw);

     int
     rw_tryenter(krwlock_t *rw, const krw_t op);

     int
     rw_tryupgrade(krwlock_t *rw);

     void
     rw_downgrade(krwlock_t *rw);

     int
     rw_read_held(krwlock_t *rw);

     int
     rw_write_held(krwlock_t *rw);

     int
     rw_lock_held(krwlock_t *rw);

     options DIAGNOSTIC
     options LOCKDEBUG

DESCRIPTION
     Reader / writer locks (RW locks) are used in the kernel to synchronize access to an object
     among LWPs (lightweight processes) and soft interrupt handlers.

     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.  Multiple callers may
		   hold a RW lock with ``read intent'' simultaneously.

     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 handler.

OPTIONS AND MACROS
     options DIANOSTIC

	   Kernels compiled with the DIAGNOSTIC option perform basic sanity checks on RW lock
	   operations.

     options LOCKDEBUG

	   Kernels compiled with the LOCKDEBUG option perform potentially CPU intensive sanity
	   checks on RW lock operations.

FUNCTIONS
     rw_init(rw)

	   Initialize a lock for use.  No other operations can be performed on the lock until it
	   has been initialized.

     rw_destroy(rw)

	   Release resources used by a lock.  The lock may not be used after it has been
	   destroyed.

     rw_enter(rw, op)

	   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 is acquired.

	   RW locks 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.

     rw_exit(rw)

	   Release a lock.  The lock must have been previously acquired by the caller.

     rw_tryenter(rw, op)

	   Try to acquire a lock, but do not block if the lock is already held.  If the lock is
	   acquired successfully, return non-zero.  Otherwise, return zero.

	   Valid arguments to op are RW_READER or RW_WRITER.

     rw_tryupgrade(rw)

	   Try to upgrade a lock from one read hold to a write hold.  If the lock is upgraded
	   successfully, returns non-zero.  Otherwise, returns zero.

     rw_downgrade(rw)

	   Downgrade a lock from a write hold to a read hold.

     rw_write_held(rw)

     rw_read_held(rw)

     rw_lock_held(rw)

	   Test the lock's condition and return non-zero if the lock is held (potentially by the
	   current LWP) and matches the specified condition.  Otherwise, return zero.

	   These functions must never be used to make locking decisions at run time: they are
	   provided only for diagnostic purposes.

PERFORMANCE CONSIDERATIONS
     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 following three conditions are met:

     o	 The data object(s) protected by the RW lock are read much more frequently than written.

     o	 The read-side hold time for the RW lock is long (in the order of thousands of processor
	 clock cycles).

     o	 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 correct operation.

CODE REFERENCES
     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.

SEE ALSO
     lockstat(8), condvar(9), mb(9), mutex(9)

     Jim Mauro and Richard McDougall, Solaris Internals: Core Kernel Architecture, Prentice Hall,
     2001, ISBN 0-13-022496-0.

HISTORY
     The RW lock primitives first appeared in NetBSD 5.0.

BSD					November 22, 2009				      BSD


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