CRITICAL_ENTER(9)      FreeBSD Kernel Developer's Manual     CRITICAL_ENTER(9)

NAME

     critical_enter, critical_exit - enter and exit a critical region

SYNOPSIS

     #include <sys/param.h>
     #include <sys/systm.h>

     void
     critical_enter(void);

     void
     critical_exit(void);

     CRITICAL_ASSERT(struct thread *td);

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, 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.

     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.

     The critical_enter() and critical_exit() functions manage a per-thread
     counter to handle nested critical sections.  If a thread is made runnable
     that would normally preempt the current thread while the current thread
     is in a critical section, then the preemption will be deferred until the
     current thread exits the outermost critical section.

     Note that these functions do not provide any inter-CPU synchronization,
     data protection, or memory ordering guarantees, and thus should not be
     used to protect shared data structures.

     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, 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.

     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 callout(9) events on the current CPU.

     The CRITICAL_ASSERT() macro verifies that the provided thread td is
     currently executing in a critical section.  It is a wrapper around
     KASSERT(9).

SEE ALSO

     callout(9), KASSERT(9), locking(9)

HISTORY

     These functions were introduced in FreeBSD 5.0.

FreeBSD 15.1-STABLE-HBSD        March 20, 2023               CRITICAL_ENTER(9)