1.\" Copyright (c) 2001,2002 John H. Baldwin <jhb@FreeBSD.org> 2.\" 3.\" Redistribution and use in source and binary forms, with or without 4.\" modification, are permitted provided that the following conditions 5.\" are met: 6.\" 1. Redistributions of source code must retain the above copyright 7.\" notice, this list of conditions and the following disclaimer. 8.\" 2. Redistributions in binary form must reproduce the above copyright 9.\" notice, this list of conditions and the following disclaimer in the 10.\" documentation and/or other materials provided with the distribution. 11.\" 12.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 13.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 14.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 15.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 16.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 17.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 18.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 19.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 20.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 21.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 22.\" SUCH DAMAGE. 23.\" 24.\" $FreeBSD$ 25.\" 26.Dd March 20, 2023 27.Dt CRITICAL_ENTER 9 28.Os 29.Sh NAME 30.Nm critical_enter , 31.Nm critical_exit 32.Nd enter and exit a critical region 33.Sh SYNOPSIS 34.In sys/param.h 35.In sys/systm.h 36.Ft void 37.Fn critical_enter "void" 38.Ft void 39.Fn critical_exit "void" 40.Fn CRITICAL_ASSERT "struct thread *td" 41.Sh DESCRIPTION 42These functions are used to prevent preemption in a critical region of code. 43All that is guaranteed is that the thread currently executing on a CPU will 44not be preempted. 45Specifically, a thread in a critical region will not migrate to another CPU 46while it is in a critical region, nor will the current CPU switch to a 47different thread. 48The current CPU may still trigger faults and exceptions during a critical 49section; however, these faults are usually fatal. 50.Pp 51The CPU might also receive and handle interrupts within a critical section. 52When this occurs the interrupt exit will not result in a context switch, and 53execution will continue in the critical section. 54Thus, the net effect of a critical section on the current thread's execution is 55similar to running with interrupts disabled, except that timer interrupts and 56filtered interrupt handlers do not incur a latency penalty. 57.Pp 58The 59.Fn critical_enter 60and 61.Fn critical_exit 62functions manage a per-thread counter to handle nested critical sections. 63If a thread is made runnable that would normally preempt the current thread 64while the current thread is in a critical section, 65then the preemption will be deferred until the current thread exits the 66outermost critical section. 67.Pp 68Note that these functions do not provide any inter-CPU synchronization, data 69protection, or memory ordering guarantees, and thus should 70.Em not 71be used to protect shared data structures. 72.Pp 73These functions should be used with care as an unbound or infinite loop within 74a critical region will deadlock the CPU. 75Also, they should not be interlocked with operations on mutexes, sx locks, 76semaphores, or other synchronization primitives, as these primitives may 77require a context switch to operate. 78One exception to this is that spin mutexes include a critical section, 79so in certain cases critical sections may be interlocked with spin mutexes. 80.Pp 81Critical regions should be only as wide as necessary. 82That is, code which does not require the critical section to operate correctly 83should be excluded from its bounds whenever possible. 84Abuse of critical sections has an effect on overall system latency and timer 85precision, since disabling preemption will delay the execution of threaded 86interrupt handlers and 87.Xr callout 9 88events on the current CPU. 89.Pp 90The 91.Fn CRITICAL_ASSERT 92macro verifies that the provided thread 93.Fa td 94is currently executing in a critical section. 95It is a wrapper around 96.Xr KASSERT 9 . 97.Sh SEE ALSO 98.Xr callout 9 , 99.Xr KASSERT 9 , 100.Xr locking 9 101.Sh HISTORY 102These functions were introduced in 103.Fx 5.0 . 104