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Add the upcoming atomic_fcmpset family to the atomic(9) man page.

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These primitives give the caller the read value if the exchange attempt
failed which saves an explicit reload for cmpset loops.

The man page was partially submitted by kib.

Reviewed by:	kib (previous version), jhb (previous version)
This commit is contained in:
Mateusz Guzik 2017-01-03 20:59:50 +00:00
parent fa699bb23e
commit c0b995bb1b
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=311168
1 changed files with 57 additions and 1 deletions
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58
share/man/man9/atomic.9
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@ -23,13 +23,14 @@
.\"
.\" $FreeBSD$
.\"
.Dd May 12, 2016
.Dd Jan 3, 2017
.Dt ATOMIC 9
.Os
.Sh NAME
.Nm atomic_add ,
.Nm atomic_clear ,
.Nm atomic_cmpset ,
.Nm atomic_fcmpset ,
.Nm atomic_fetchadd ,
.Nm atomic_load ,
.Nm atomic_readandclear ,
@ -50,6 +51,12 @@
.Fa "<type> old"
.Fa "<type> new"
.Fc
.Ft int
.Fo atomic_fcmpset_[acq_|rel_]<type>
.Fa "volatile <type> *dst"
.Fa "<type> *old"
.Fa "<type> new"
.Fc
.Ft <type>
.Fn atomic_fetchadd_<type> "volatile <type> *p" "<type> v"
.Ft <type>
@ -228,6 +235,45 @@ functions are not implemented for the types
and
.Dq Li 16 .
.Bl -hang
.It Fn atomic_fcmpset dst *old new
.El
.Pp
On architectures implementing
.Em Compare And Swap
operation in hardware, the functionality can be described as
.Bd -literal -offset indent -compact
if (*dst == *old) {
*dst = new;
return (1);
} else {
*old = *dst;
return (0);
}
.Ed
On architectures which provide
.Em Load Linked/Store Conditional
primitive, the write to
.Dv *dst
might also fail for several reasons, most important of which
is a parallel write to
.Dv *dst
cache line by other CPU.
In this case
.Fn atomic_fcmpset
function also returns
.Dv false ,
despite
.Dl *old == *dst .
.Pp
The
.Fn atomic_fcmpset
functions are not implemented for the types
.Dq Li char ,
.Dq Li short ,
.Dq Li 8 ,
and
.Dq Li 16 .
.Bl -hang
.It Fn atomic_fetchadd p v
.Bd -literal -compact
tmp = *p;
@ -353,6 +399,16 @@ The
.Fn atomic_cmpset
function returns the result of the compare operation.
The
.Fn atomic_fcmpset
function returns
.Dv true
if the operationg succeeded.
Otherwise it returns
.Dv false
and sets
.Va *old
to the found value.
The
.Fn atomic_fetchadd ,
.Fn atomic_load ,
.Fn atomic_readandclear ,