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.Dd $Mdocdate: March 13 2022 $
.Dt MEMBAR 9
.Os
.Sh NAME
.Nm membar_enter ,
.Nm membar_exit ,
.Nm membar_producer ,
.Nm membar_consumer ,
.Nm membar_sync ,
.Nm membar_enter_after_atomic ,
.Nm membar_exit_before_atomic
.Nd memory access barrier operations
.Sh SYNOPSIS
.In sys/atomic.h
.Ft void
.Fn membar_enter "void"
.Ft void
.Fn membar_exit "void"
.Ft void
.Fn membar_producer "void"
.Ft void
.Fn membar_consumer "void"
.Ft void
.Fn membar_sync "void"
.Ft void
.Fn membar_enter_after_atomic "void"
.Ft void
.Fn membar_exit_before_atomic "void"
.Sh DESCRIPTION
The membar set of functions provide an interface for issuing memory barrier
access operations with respect to multiple processors in the system.
.Bl -tag -width "mem"
.It Fn membar_enter
Any store preceding
.Fn membar_enter
will reach global visibility before all loads and stores following it.
.Pp
.Fn membar_enter
is typically used in code that implements locking primitives to ensure
that a lock protects its data.
.It Fn membar_exit
All loads and stores preceding
.Fn membar_exit
will reach global visibility before any store that follows it.
.Pp
.Fn membar_exit
is typically used in code that implements locking primitives to ensure
that a lock protects its data.
.It Fn membar_producer
All stores preceding the memory barrier will reach global visibility
before any stores after the memory barrier reach global visibility.
.It Fn membar_consumer
All loads preceding the memory barrier will complete before any loads
after the memory barrier complete.
.It Fn membar_sync
All loads and stores preceding the memory barrier will complete and
reach global visibility before any loads and stores after the memory
barrier complete and reach global visibility.
.It Fn membar_enter_after_atomic
An atomic operation preceding
.Fn membar_enter_after_atomic
will reach global visibility before all loads and stores following it.
The atomic operation is used to protect the start of a critical section.
.It Fn membar_exit_before_atomic
All loads and stores preceding
.Fn membar_exit_before_atomic
will reach global visibility before atomic operation that follows it.
The atomic operation is used to protect the end of a critical section.
.El
.Pp
The atomic operations that can be used with
.Fn membar_enter_after_atomic
and
.Fn membar_exit_before_atomic
are the atomic_add, atomic_sub, atomic_inc, atomic_dec, and atomic_cas
set of functions.
For other cases use
.Fn membar_enter
or
.Fn membar_exit .
.Sh CONTEXT
.Fn membar_enter ,
.Fn membar_exit ,
.Fn membar_producer ,
.Fn membar_consumer ,
.Fn membar_sync ,
.Fn membar_enter_after_atomic ,
.Fn membar_exit_before_atomic
can all be called during autoconf, from process context, or from
interrupt context.
.Sh HISTORY
The membar functions first appeared in
.Nx 5.0
and
.Ox 5.5 .