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32 .Nd introduction to kernel locking primitives
36 kernel provides several locking and synchronisation primitives, each with
37 different characteristics and purposes. This manpage aims at giving an
38 overview of the available locking primitives and their usecases.
39 .Sh CONDITION VARIABLES
40 Condition variables are used to wait for conditions to occur. In
42 condition variables use spinlocks internally.
43 Threads that wait on a condition variable are called waiters. Either just
44 one or all waiters can be notified of changes to a condition variable.
45 A condition variable can interlock sleep when given a lockmgr lock to
46 avoid missing changes to it, or regular tsleep.
52 A critical section changes the priority of the current thread to
53 TDPRIT_CRIT, effectively avoiding preemption of the thread.
54 Critical sections are a per-cpu primitive, and there is no synchronisation
55 or locking between CPUs.
60 Lockmgr locks are the kitchen sink locking primitive for the
64 for more information. Lockmgr locks should be used for FreeBSD compatibility when porting drivers that use FreeBSD's mutexes.
69 .Sh LWKT SERIALIZING TOKENS
71 Lwkt serializing tokens use atomic_cmpset internally and are integrated with
72 the lwkt serializer. The scheduler takes care of acquiring a token before
73 rescheduling, so a thread will not be run unless all tokens for it can be
76 Tokens are not owned by a thread, but by the CPU, and threads are only given
82 The mplock is an API wrapper for the MP token. The use of this should be
83 avoided at all cost, because there is only one MP token for the whole system.
86 Mtx mutexes are a locking primitive that is based around atomic_cmpset_int instead
87 of spinlocks. They are much faster and use less memory than than lockmgr locks.
88 Mtx can always be recursive, shared/exclusive and can be held across blocking calls
90 Mtx are also capable of passing ownership directly to a new owner without wakeup.
94 Serializers are used to serialize access to hardware and other subsystems.
95 Serializers are deprecated, and should not be used in new code.
98 Spinlocks employ a busy wait loop to acquire a lock. This means that this type of lock
99 is very lightweight, but should only be held for a very short time, since all contenders
100 will be spinning and not sleeping. No wakeup is necessary, because a waiter will be
102 If a thread tries to sleep while holding a spinlock, the kernel will panic.
103 Spinlocks cannot recurse.
106 Spinlocks are mainly used to protect kernel structures, and to implement higher level
112 This manual page was written by
113 .An Markus Pfeiffer Aq Mt markus.pfeiffer@morphism.de ,
114 based on comments by various