RWLOCK(9)              NetBSD 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.  Mul-
                   tiple 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 con-
           sistent 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.  Oth-
           erwise, 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 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.

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 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
     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.

NetBSD 6.1.3                   November 22, 2009                  NetBSD 6.1.3

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