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critical(9): small updates

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- Document CRITICAL_ASSERT() in this man page.
 - Clarify that a thread may also handle interrupts in a critical
   section, not only faults/exceptions.
 - Note the negative effects of critical section abuse
 - Some other minor clarifications
 - Add short SEE ALSO

Reviewed by:	kib, markj, rpokala, Pau Amma <pauamma@gundo.com>
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D39130
This commit is contained in:
Mitchell Horne 2023-03-20 16:50:50 -03:00
parent b54391a1f8
commit 43db15b16a
2 changed files with 41 additions and 10 deletions
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3
share/man/man9/Makefile
View File

@ -929,7 +929,8 @@ MLINKS+=cpuset.9 CPUSET_T_INITIALIZER.9 \
cpuset.9 CPU_OR_ATOMIC.9 \
cpuset.9 CPU_COPY_STORE_REL.9
MLINKS+=critical_enter.9 critical.9 \
critical_enter.9 critical_exit.9
critical_enter.9 critical_exit.9 \
critical_enter.9 CRITICAL_ASSERT.9
MLINKS+=crypto_buffer.9 crypto_apply.9 \
crypto_buffer.9 crypto_apply_buf.9 \
crypto_buffer.9 crypto_buffer_contiguous_segment.9 \

48
share/man/man9/critical_enter.9
View File

@ -23,7 +23,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd October 5, 2005
.Dd March 20, 2023
.Dt CRITICAL_ENTER 9
.Os
.Sh NAME
@ -37,15 +37,24 @@
.Fn critical_enter "void"
.Ft void
.Fn critical_exit "void"
.Fn CRITICAL_ASSERT "struct thread *td"
.Sh DESCRIPTION
These functions are used to prevent preemption in a critical region of code.
All that is guaranteed is that the thread currently executing on a CPU will
not be preempted.
Specifically, a thread in a critical region will not migrate to another
CPU while it is in a critical region.
Specifically, a thread in a critical region will not migrate to another CPU
while it is in a critical region, nor will the current CPU switch to a
different thread.
The current CPU may still trigger faults and exceptions during a critical
section; however, these faults are usually fatal.
.Pp
The CPU might also receive and handle interrupts within a critical section.
When this occurs the interrupt exit will not result in a context switch, and
execution will continue in the critical section.
Thus, the net effect of a critical section on the current thread's execution is
similar to running with interrupts disabled, except that timer interrupts and
filtered interrupt handlers do not incur a latency penalty.
.Pp
The
.Fn critical_enter
and
@ -56,18 +65,39 @@ while the current thread is in a critical section,
then the preemption will be deferred until the current thread exits the
outermost critical section.
.Pp
Note that these functions are not required to provide any inter-CPU
synchronization, data protection, or memory ordering guarantees and thus
should
Note that these functions do not provide any inter-CPU synchronization, data
protection, or memory ordering guarantees, and thus should
.Em not
be used to protect shared data structures.
.Pp
These functions should be used with care as an infinite loop within a
critical region will deadlock the CPU.
These functions should be used with care as an unbound or infinite loop within
a critical region will deadlock the CPU.
Also, they should not be interlocked with operations on mutexes, sx locks,
semaphores, or other synchronization primitives.
semaphores, or other synchronization primitives, as these primitives may
require a context switch to operate.
One exception to this is that spin mutexes include a critical section,
so in certain cases critical sections may be interlocked with spin mutexes.
.Pp
Critical regions should be only as wide as necessary.
That is, code which does not require the critical section to operate correctly
should be excluded from its bounds whenever possible.
Abuse of critical sections has an effect on overall system latency and timer
precision, since disabling preemption will delay the execution of threaded
interrupt handlers and
.Xr callout 9
events on the current CPU.
.Pp
The
.Fn CRITICAL_ASSERT
macro verifies that the provided thread
.Fa td
is currently executing in a critical section.
It is a wrapper around
.Xr KASSERT 9 .
.Sh SEE ALSO
.Xr callout 9 ,
.Xr KASSERT 9 ,
.Xr locking 9
.Sh HISTORY
These functions were introduced in
.Fx 5.0 .