1.\" $NetBSD: rwlock.9,v 1.9 2007/12/05 05:22:22 ad Exp $ 2.\" 3.\" Copyright (c) 2006, 2007 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Andrew Doran. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the NetBSD 20.\" Foundation, Inc. and its contributors. 21.\" 4. Neither the name of The NetBSD Foundation nor the names of its 22.\" contributors may be used to endorse or promote products derived 23.\" from this software without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 26.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 27.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 28.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 29.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 30.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 31.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 32.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 33.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 34.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 35.\" POSSIBILITY OF SUCH DAMAGE. 36.\" 37.Dd December 4, 2007 38.Dt RWLOCK 9 39.Os 40.Sh NAME 41.Nm rw , 42.Nm rw_init , 43.Nm rw_destroy , 44.Nm rw_enter , 45.Nm rw_exit , 46.Nm rw_tryenter , 47.Nm rw_tryupgrade , 48.Nm rw_downgrade , 49.Nm rw_read_held , 50.Nm rw_write_held , 51.Nm rw_lock_held 52.Nd reader / writer lock primitives 53.Sh SYNOPSIS 54.In sys/rwlock.h 55.Ft void 56.Fn rw_init "krwlock_t *rw" 57.Ft void 58.Fn rw_destroy "krwlock_t *rw" 59.Ft void 60.Fn rw_enter "krwlock_t *rw" "const krw_t op" 61.Ft void 62.Fn rw_exit "krwlock_t *rw" 63.Ft int 64.Fn rw_tryenter "krwlock_t *rw" "const krw_t op" 65.Ft int 66.Fn rw_tryupgrade "krwlock_t *rw" 67.Ft void 68.Fn rw_downgrade "krwlock_t *rw" 69.Ft int 70.Fn rw_read_held "krwlock_t *rw" 71.Ft int 72.Fn rw_write_held "krwlock_t *rw" 73.Ft int 74.Fn rw_lock_held "krwlock_t *rw" 75.Pp 76.Cd "options DIAGNOSTIC" 77.Cd "options LOCKDEBUG" 78.Sh DESCRIPTION 79Reader / writer locks (RW locks) are used in the kernel to synchronize access 80to an object among LWPs (lightweight processes) and soft interrupt handlers. 81.Pp 82In addition to the capabilities provided by mutexes, RW locks distinguish 83between read (shared) and write (exclusive) access. 84RW locks are intended to provide protection for kernel data 85or objects that are read much more frequently than updated. 86For objects that are updated as frequently as they are read, mutexes should 87be used to guarantee atomic access. 88.Pp 89RW locks are in one of three distinct states at any given time: 90.Bl -tag -width cdosrunrundo 91.It Dv Unlocked 92The lock is not held. 93.It Dv Read locked 94The lock holders intend to read the protected object. 95Multiple callers may hold a RW lock with 96.Dq read intent 97simultaneously. 98.It Dv Write locked 99The lock holder intends to update the protected object. 100Only one caller may hold a RW lock with 101.Dq write intent . 102.El 103.Pp 104The 105.Vt krwlock_t 106type provides storage for the RW lock object. 107This should be treated as an opaque object and not examined directly by 108consumers. 109.Pp 110Note that the these interfaces must not be used from a hardware 111interrupt handler. 112.Sh OPTIONS AND MACROS 113.Bl -tag -width abcd 114.It Cd "options DIANOSTIC" 115.Pp 116Kernels compiled with the 117.Dv DIAGNOSTIC 118option perform basic sanity checks on RW lock operations. 119.It Cd "options LOCKDEBUG" 120.Pp 121Kernels compiled with the 122.Dv LOCKDEBUG 123option perform potentially CPU intensive sanity checks 124on RW lock operations. 125.El 126.Sh FUNCTIONS 127.Bl -tag -width abcd 128.It Fn rw_init "rw" 129.Pp 130Initialize a lock for use. 131No other operations can be performed on the lock until it has been 132initialized. 133.It Fn rw_destroy "rw" 134.Pp 135Release resources used by a lock. 136The lock may not be used after it has been destroyed. 137.It Fn rw_enter "rw" "op" 138.Pp 139If 140.Dv RW_READER 141is specified as the argument to 142.Fa op , 143acquire a read lock. 144If the lock is write held, the caller will block and not return until the 145hold is acquired. 146Callers must not recursively acquire read locks. 147.Pp 148If 149.Dv RW_WRITER 150is specified, acquire a write lock. 151If the lock is already held, the caller will block and not return until the 152hold is acquired. 153.Pp 154RW locks and other types of locks must always be acquired in a 155consistent order with respect to each other. 156Otherwise, the potential for system deadlock exists. 157.It Fn rw_exit "rw" 158.Pp 159Release a lock. 160The lock must have been previously acquired by the caller. 161.It Fn rw_tryenter "rw" "op" 162.Pp 163Try to acquire a lock, but do not block if the lock is already held. 164If the lock is acquired successfully, return non-zero. 165Otherwise, return zero. 166.Pp 167Valid arguments to 168.Fa op 169are 170.Dv RW_READER 171or 172.Dv RW_WRITER . 173.It Fn rw_tryupgrade "rw" 174.Pp 175Try to upgrade a lock from one read hold to a write hold. 176If the lock is upgraded successfully, returns non-zero. 177Otherwise, returns zero. 178.It Fn rw_downgrade "rw" 179.Pp 180Downgrade a lock from a write hold to a read hold. 181.It Fn rw_write_held "rw" 182.It Fn rw_read_held "rw" 183.It Fn rw_lock_held "rw" 184.Pp 185Test the lock's condition and return non-zero if the lock is held 186(potentially by the current LWP) and matches the specified condition. 187Otherwise, return zero. 188.Pp 189These functions must never be used to make locking decisions at run time: 190they are provided only for diagnostic purposes. 191.El 192.Sh CODE REFERENCES 193This section describes places within the 194.Nx 195source tree where code implementing RW locks can be found. 196All pathnames are relative to 197.Pa /usr/src . 198.Pp 199The core of the RW lock implementation is in 200.Pa sys/kern/kern_rwlock.c . 201.Pp 202The header file 203.Pa sys/sys/rwlock.h 204describes the public interface, and interfaces that machine-dependent 205code must provide to support RW locks. 206.Sh SEE ALSO 207.Xr condvar 9 , 208.Xr mb 9 , 209.Xr mutex 9 210.Rs 211.%A Jim Mauro 212.%A Richard McDougall 213.%T Solaris Internals: Core Kernel Architecture 214.%I Prentice Hall 215.%D 2001 216.%O ISBN 0-13-022496-0 217.Re 218.Sh HISTORY 219The RW lock primitives first appeared in 220.Nx 5.0 . 221