checkpatch: whine about ACCESS_ONCE

Add a test for use of ACCESS_ONCE that could be written using READ_ONCE or
WRITE_ONCE.

--fix it too if desired.

The WRITE_ONCE fixes are less correct than the coccinelle script below as
checkpatch cannot have a completely correct "expression" mechanism because
checkpatch works on patches and not complete files.

$ cat access_once.cocci
@@
expression e1;
expression e2;
@@

-       ACCESS_ONCE(e1) = e2
+       WRITE_ONCE(e1, e2)

@@
expression e1;
@@

-       ACCESS_ONCE(e1)
+       READ_ONCE(e1)
Signed-off-by: NJoe Perches <joe@perches.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -5850,6 +5850,28 @@ sub process {
 			}
 		}
 
+# whine about ACCESS_ONCE
+		if ($^V && $^V ge 5.10.0 &&
+		    $line =~ /\bACCESS_ONCE\s*$balanced_parens\s*(=(?!=))?\s*($FuncArg)?/) {
+			my $par = $1;
+			my $eq = $2;
+			my $fun = $3;
+			$par =~ s/^\(\s*(.*)\s*\)$/$1/;
+			if (defined($eq)) {
+				if (WARN("PREFER_WRITE_ONCE",
+					 "Prefer WRITE_ONCE(<FOO>, <BAR>) over ACCESS_ONCE(<FOO>) = <BAR>\n" . $herecurr) &&
+				    $fix) {
+					$fixed[$fixlinenr] =~ s/\bACCESS_ONCE\s*\(\s*\Q$par\E\s*\)\s*$eq\s*\Q$fun\E/WRITE_ONCE($par, $fun)/;
+				}
+			} else {
+				if (WARN("PREFER_READ_ONCE",
+					 "Prefer READ_ONCE(<FOO>) over ACCESS_ONCE(<FOO>)\n" . $herecurr) &&
+				    $fix) {
+					$fixed[$fixlinenr] =~ s/\bACCESS_ONCE\s*\(\s*\Q$par\E\s*\)/READ_ONCE($par)/;
+				}
+			}
+		}
+
 # check for lockdep_set_novalidate_class
 		if ($line =~ /^.\s*lockdep_set_novalidate_class\s*\(/ ||
 		    $line =~ /__lockdep_no_validate__\s*\)/ ) {