Chapter 2. Locking Notes

Table of Contents
2.1. Mutexes
2.2. Shared Exclusive Locks
2.3. Atomically Protected Variables

This chapter is maintained by the FreeBSD SMP Next Generation Project.

This document outlines the locking used in the FreeBSD kernel to permit effective multi-processing within the kernel. Locking can be achieved via several means. Data structures can be protected by mutexes or lockmgr(9) locks. A few variables are protected simply by always using atomic operations to access them.

2.1. Mutexes

A mutex is simply a lock used to guarantee mutual exclusion. Specifically, a mutex may only be owned by one entity at a time. If another entity wishes to obtain a mutex that is already owned, it must wait until the mutex is released. In the FreeBSD kernel, mutexes are owned by processes.

Mutexes may be recursively acquired, but they are intended to be held for a short period of time. Specifically, one may not sleep while holding a mutex. If you need to hold a lock across a sleep, use a lockmgr(9) lock.

Each mutex has several properties of interest:

Variable Name

The name of the struct mtx variable in the kernel source.

Logical Name

The name of the mutex assigned to it by mtx_init. This name is displayed in KTR trace messages and witness errors and warnings and is used to distinguish mutexes in the witness code.


The type of the mutex in terms of the MTX_* flags. The meaning for each flag is related to its meaning as documented in mutex(9).


A sleep mutex


A spin mutex


This mutex is allowed to recurse.


A list of data structures or data structure members that this entry protects. For data structure members, the name will be in the form of structure name.member name.

Dependent Functions

Functions that can only be called if this mutex is held.

Table 2.1. Mutex List
Variable NameLogical NameTypeProtecteesDependent Functions
sched_lock sched lock MTX_SPIN | MTX_RECURSE _gmonparam, cnt.v_swtch, cp_time, curpriority, mtx.mtx_blocked, mtx.mtx_contested, proc.p_procq, proc.p_slpq, proc.p_sflag, proc.p_stat, proc.p_estcpu, proc.p_cpticks proc.p_pctcpu, proc.p_wchan, proc.p_wmesg, proc.p_swtime, proc.p_slptime, proc.p_runtime, proc.p_uu, proc.p_su, proc.p_iu, proc.p_uticks, proc.p_sticks, proc.p_iticks, proc.p_oncpu, proc.p_lastcpu, proc.p_rqindex, proc.p_heldmtx, proc.p_blocked, proc.p_mtxname, proc.p_contested, proc.p_priority, proc.p_usrpri, proc.p_nativepri, proc.p_nice, proc.p_rtprio, pscnt, slpque, itqueuebits, itqueues, rtqueuebits, rtqueues, queuebits, queues, idqueuebits, idqueues, switchtime, switchticks setrunqueue, remrunqueue, mi_switch, chooseproc, schedclock, resetpriority, updatepri, maybe_resched, cpu_switch, cpu_throw, need_resched, resched_wanted, clear_resched, aston, astoff, astpending, calcru, proc_compare
vm86pcb_lockvm86pcb lock MTX_DEF vm86pcb vm86_bioscall
GiantGiant MTX_DEF | MTX_RECURSEnearly everythinglots
callout_lockcallout lock MTX_SPIN | MTX_RECURSE callfree, callwheel, nextsoftcheck, proc.p_itcallout, proc.p_slpcallout, softticks, ticks 

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