Module Name: src
Committed By: riastradh
Date: Tue Jun 18 19:10:50 UTC 2024
Modified Files:
src/share/man/man9: atomic_loadstore.9
Log Message:
atomic_loadstore(9): Clarify relation to __HAVE_ATOMIC64_OPS.
PR kern/58340
To generate a diff of this commit:
cvs rdiff -u -r1.7 -r1.8 src/share/man/man9/atomic_loadstore.9
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/share/man/man9/atomic_loadstore.9
diff -u src/share/man/man9/atomic_loadstore.9:1.7 src/share/man/man9/atomic_loadstore.9:1.8
--- src/share/man/man9/atomic_loadstore.9:1.7 Sat Apr 9 23:32:53 2022
+++ src/share/man/man9/atomic_loadstore.9 Tue Jun 18 19:10:50 2024
@@ -1,4 +1,4 @@
-.\" $NetBSD: atomic_loadstore.9,v 1.7 2022/04/09 23:32:53 riastradh Exp $
+.\" $NetBSD: atomic_loadstore.9,v 1.8 2024/06/18 19:10:50 riastradh Exp $
.\"
.\" Copyright (c) 2019 The NetBSD Foundation
.\" All rights reserved.
@@ -544,9 +544,10 @@ must be zero.
.Pp
All
.Nx
-ports support atomic loads and stores on units of data up to 32 bits.
-Some ports additionally support atomic loads and stores on larger
-quantities, like 64-bit quantities, if
+ports support cheap atomic loads and stores on units of data up to 32
+bits.
+Some ports additionally support cheap atomic loads and stores on
+64-bit quantities if
.Dv __HAVE_ATOMIC64_LOADSTORE
is defined.
The macros are not allowed on larger quantities of data than the port
@@ -587,6 +588,24 @@ must be conditional on
...
mutex_exit(&foo->f_lock);
.Ed
+.Pp
+Some ports support expensive 64-bit atomic read/modify/write
+operations, but not cheap 64-bit atomic loads and stores.
+For example, the armv7 instruction set includes 64-bit
+.Li ldrexd
+and
+.Li strexd
+loops (load-exclusive, store-conditional) which are atomic on 64-bit
+quantities.
+But the cheap 64-bit
+.Li ldrd / strd
+instructions are only atomic on 32-bit accesses at a time.
+These ports define
+.Dv __HAVE_ATOMIC64_OPS
+but not
+.Dv __HAVE_ATOMIC64_LOADSTORE ,
+since they do not have cheaper 64-bit atomic load/store operations than
+the full atomic read/modify/write operations.
.Sh C11 COMPATIBILITY
These macros are meant to follow
.Tn C11
