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.Dd March 20, 2023 25.Dt CRITICAL_ENTER 9 26.Os 27.Sh NAME 28.Nm critical_enter , 29.Nm critical_exit 30.Nd enter and exit a critical region 31.Sh SYNOPSIS 32.In sys/param.h 33.In sys/systm.h 34.Ft void 35.Fn critical_enter "void" 36.Ft void 37.Fn critical_exit "void" 38.Fn CRITICAL_ASSERT "struct thread *td" 39.Sh DESCRIPTION 40These functions are used to prevent preemption in a critical region of code. 41All that is guaranteed is that the thread currently executing on a CPU will 42not be preempted. 43Specifically, a thread in a critical region will not migrate to another CPU 44while it is in a critical region, nor will the current CPU switch to a 45different thread. 46The current CPU may still trigger faults and exceptions during a critical 47section; however, these faults are usually fatal. 48.Pp 49The CPU might also receive and handle interrupts within a critical section. 50When this occurs the interrupt exit will not result in a context switch, and 51execution will continue in the critical section. 52Thus, the net effect of a critical section on the current thread's execution is 53similar to running with interrupts disabled, except that timer interrupts and 54filtered interrupt handlers do not incur a latency penalty. 55.Pp 56The 57.Fn critical_enter 58and 59.Fn critical_exit 60functions manage a per-thread counter to handle nested critical sections. 61If a thread is made runnable that would normally preempt the current thread 62while the current thread is in a critical section, 63then the preemption will be deferred until the current thread exits the 64outermost critical section. 65.Pp 66Note that these functions do not provide any inter-CPU synchronization, data 67protection, or memory ordering guarantees, and thus should 68.Em not 69be used to protect shared data structures. 70.Pp 71These functions should be used with care as an unbound or infinite loop within 72a critical region will deadlock the CPU. 73Also, they should not be interlocked with operations on mutexes, sx locks, 74semaphores, or other synchronization primitives, as these primitives may 75require a context switch to operate. 76One exception to this is that spin mutexes include a critical section, 77so in certain cases critical sections may be interlocked with spin mutexes. 78.Pp 79Critical regions should be only as wide as necessary. 80That is, code which does not require the critical section to operate correctly 81should be excluded from its bounds whenever possible. 82Abuse of critical sections has an effect on overall system latency and timer 83precision, since disabling preemption will delay the execution of threaded 84interrupt handlers and 85.Xr callout 9 86events on the current CPU. 87.Pp 88The 89.Fn CRITICAL_ASSERT 90macro verifies that the provided thread 91.Fa td 92is currently executing in a critical section. 93It is a wrapper around 94.Xr KASSERT 9 . 95.Sh SEE ALSO 96.Xr callout 9 , 97.Xr KASSERT 9 , 98.Xr locking 9 99.Sh HISTORY 100These functions were introduced in 101.Fx 5.0 . 102