--- /sys/src/libc/arm/atom.s	Sun May 29 14:16:19 2016
+++ /sys/src/libc/arm/atom.s	Tue Jan 11 17:24:19 2022
@@ -31,6 +31,7 @@
 
 TEXT _xinc(SB), $0	/* void	_xinc(long *); */
 TEXT ainc(SB), $0	/* long ainc(long *); */
+	DMB
 spinainc:
 	LDREX(0,3)	/*	LDREX	0(R0),R3	*/
 	ADD	$1,R3
@@ -44,6 +45,7 @@
 
 TEXT _xdec(SB), $0	/* long _xdec(long *); */
 TEXT adec(SB), $0	/* long adec(long *); */
+	DMB
 spinadec:
 	LDREX(0,3)	/*	LDREX	0(R0),R3	*/
 	SUB	$1,R3
--- /sys/man/2/ainc	Mon Jun  7 20:16:50 2021
+++ /sys/man/2/ainc	Tue Jan 11 19:02:26 2022
@@ -41,6 +41,13 @@
 increment and decrement (respectively) the integer pointed to by
 .IR p ,
 and return the resulting value after the operation.
+On architectures with a weak memory model, both operations
+are implemented with a barrier to memory reordering.
+In other words, all loads and stores
+which precede the increment / decrement in program order
+will be observed before it,
+and the increment / decrement will be observed before any loads
+and stores which follow it in program order.
 .PP
 .I Cas
 performs a compare and swap on the word pointed to by