1.\" 2.\" Copyright (c) 2014 Markus Pfeiffer 3.\" All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.\" 27.Dd June 5, 2014 28.Dt LOCKING 9 29.Os 30.Sh NAME 31.Nm locking 32.Nd introduction to kernel locking primitives 33.Sh DESCRIPTION 34The 35.Dx 36kernel provides several locking and synchronisation primitives, each with 37different characteristics and purposes. 38This manpage aims at giving an 39overview of the available locking primitives and their use cases, as well 40as pointers towards further information. 41.Ss Condition Variables 42Condition variables are used to wait for conditions to occur. 43In 44.Dx 45condition variables use a 46.Xr spinlock 9 47internally. 48Threads that wait on a condition variable are called waiters. 49Either just one or all waiters can be notified of changes to a 50condition variable. 51A condition variable can 52.Xr tsleep_interlock 9 53when given a 54.Xr lockmgr 9 55lock to avoid missing changes to it, or use regular 56.Xr tsleep 9 . 57.Pp 58See 59.Xr condvar 9 60for further information. 61.Ss Critical Sections 62A critical section changes the priority of the current thread to 63.Dv TDPRIT_CRIT , 64effectively avoiding preemption of the thread. 65Critical sections are a per-cpu primitive, and there is no synchronisation 66or locking between CPUs. 67.Pp 68See 69.Xr crit_enter 9 . 70for further information. 71.Ss Lockmgr Locks 72.Xr Lockmgr 9 73locks are the kitchen sink locking primitive for the 74.Dx 75kernel, and the most heavyweight locking mechanism. 76.Xr lockmgr 9 77locks can be shared/exclusive and recursive. 78Lockmgr locks should be used for 79.Fx 80compatibility when porting drivers that use 81.Fx Ap s 82mutexes. 83.Pp 84See 85.Xr lockmgr 9 86for more information. 87.Ss LWKT Messages 88LWKT messages can be used to pass messages between light weight kernel 89threads in the 90.Dx 91kernel. 92LWKT mesages are sent to message ports. Every light weight kernel thread 93possesses a message port, but more can be created if necessary. 94.Pp 95See 96.Xr msgport 9 97for more information. 98.Ss LWKT Serializers 99LWKT serializers provide a fast locked-bus-cycle-based serialization 100facility. 101They are used to serialize access to hardware and other subsystems. 102Serializers were designed to provide low level exclusive locks. 103.Pp 104See 105.Xr serializer 9 . 106for more information. 107.Ss LWKT Tokens 108LWKT tokens use 109.Xr atomic_cmpset 9 110internally and are integrated with the LWKT scheduler. 111The scheduler takes care of acquiring a token before 112rescheduling, so a thread will not be run unless all tokens for it can be 113acquired. 114Tokens are not owned by a thread, but by the CPU, and threads are only given 115references to tokens. 116See 117.Xr serializer 9 . 118.Ss MPLOCK 119The mplock is an API wrapper for the MP token. The use of this should be 120avoided at all cost, because there is only one MP token for the whole system. 121.Ss MTX Mutexes 122Mtx mutexes are a locking primitive that is based around 123.Xr atomic_cmpset_int 9 124instead of spinlocks. 125They are much faster and use less memory than 126.Xr lockmgr 9 127locks. 128Mtx mutexes can always be recursive, shared/exclusive and can be held 129across blocking calls and sleeps. 130They are also capable of passing ownership directly to a new owner 131without wakeup. 132See 133.Xr mutex 9 . 134.Ss Spinlocks 135Spinlocks employ a busy wait loop to acquire a lock. 136This means that this type of lock is very lightweight, 137but should only be held for a very short time, since all contenders 138will be spinning and not sleeping. 139No wakeup is necessary, because a waiter will be spinning already. 140If a thread tries to sleep while holding a spinlock, the kernel will panic. 141Spinlocks cannot recurse. 142.Pp 143They are mainly used to protect kernel structures, and to 144implement higher level locking primitives. 145See 146.Xr spinlock 9 . 147.Sh SEE ALSO 148.Xr atomic 9 , 149.Xr condvar 9 , 150.Xr crit_enter 9 , 151.Xr lockmgr 9 , 152.Xr mutex 9 , 153.Xr serializer 9 , 154.Xr spinlock 9 , 155.Xr tsleep 9 156.Sh AUTHORS 157.An -nosplit 158This manual page was written by 159.An Markus Pfeiffer Aq Mt markus.pfeiffer@morphism.de , 160based on comments by various 161.Dx 162authors. 163