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Message-ID: <1510698116.2006.27.camel@perches.com>
Date: Tue, 14 Nov 2017 14:21:56 -0800
From: Joe Perches <joe@...ches.com>
To: paulmck@...ux.vnet.ibm.com, Mark Rutland <mark.rutland@....com>
Cc: linux-kernel@...r.kernel.org,
Arnaldo Carvalho de Melo <acme@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] perf mmap: Convert ACCESS_ONCE() to READ_ONCE()
On Tue, 2017-11-14 at 14:08 -0800, Paul E. McKenney wrote:
> On Tue, Nov 14, 2017 at 10:31:38AM +0000, Mark Rutland wrote:
> > Recently there was a treewide conversion of ACCESS_ONCE() to
> > {READ,WRITE}_ONCE(), but a new use was introduced concurrently by
> > commit:
> >
> > 1695849735752d2a ("perf mmap: Move perf_mmap and methods to separate mmap.[ch] files")
> >
> > Let's convert this over to READ_ONCE() so that we can remove the
> > ACCESS_ONCE() definitions in subsequent patches.
> >
> > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
>
> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>
> Woo-hoo! Good to see that your Coccinelle script has already done
> its magic! ;-)
Might be nice to remove all the other references too
$ git grep -w ACCESS_ONCE
Documentation/RCU/RTFP.txt: ACCESS_ONCE().
include/linux/compiler.h: * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
include/linux/compiler.h: * WRITE_ONCE or ACCESS_ONCE() in different C statements.
include/linux/compiler.h: * In contrast to ACCESS_ONCE these two macros will also work on aggregate
include/linux/compiler.h: * is also forbidden from reordering successive instances of ACCESS_ONCE(),
include/linux/compiler.h: * ACCESS_ONCE() in different C statements.
include/linux/compiler.h: * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
include/linux/compiler.h: * The major use cases of ACCESS_ONCE used to be (1) Mediating communication
include/linux/compiler.h:#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
scripts/checkpatch.pl:# whine about ACCESS_ONCE
scripts/checkpatch.pl: "Prefer WRITE_ONCE(<FOO>, <BAR>) over ACCESS_ONCE(<FOO>) = <BAR>\n" . $herecurr) &&
scripts/checkpatch.pl: "Prefer READ_ONCE(<FOO>) over ACCESS_ONCE(<FOO>)\n" . $herecurr) &&
tools/include/linux/compiler.h:#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
tools/include/linux/compiler.h: * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
tools/include/linux/compiler.h: * WRITE_ONCE or ACCESS_ONCE() in different C statements.
tools/include/linux/compiler.h: * In contrast to ACCESS_ONCE these two macros will also work on aggregate
tools/perf/util/mmap.h: u64 head = ACCESS_ONCE(pc->data_head);
---
include/linux/compiler.h | 45 ++++++++++--------------------------------
scripts/checkpatch.pl | 22 ---------------------
tools/include/linux/compiler.h | 19 ++++++++----------
3 files changed, 18 insertions(+), 68 deletions(-)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 3672353a0acd..f729154dae9b 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -215,17 +215,17 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
/*
* Prevent the compiler from merging or refetching reads or writes. The
* compiler is also forbidden from reordering successive instances of
- * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
- * compiler is aware of some particular ordering. One way to make the
- * compiler aware of ordering is to put the two invocations of READ_ONCE,
- * WRITE_ONCE or ACCESS_ONCE() in different C statements.
+ * READ_ONCE and WRITE_ONCE (see below), but only when the compiler is
+ * aware of some particular ordering. One way to make the compiler aware
+ * of ordering is to put the two invocations of READ_ONCE and WRITE_ONCE
+ * in different C statements.
*
- * In contrast to ACCESS_ONCE these two macros will also work on aggregate
- * data types like structs or unions. If the size of the accessed data
- * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
- * READ_ONCE() and WRITE_ONCE() will fall back to memcpy(). There's at
- * least two memcpy()s: one for the __builtin_memcpy() and then one for
- * the macro doing the copy of variable - '__u' allocated on the stack.
+ * These two macros will work on aggregate data types like structs or unions.
+ * If the size of the accessed data type exceeds the word size of the machine
+ * (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will fall back to
+ * memcpy(). There are at least two memcpy()s: one for the __builtin_memcpy()
+ * and then one for the macro doing the copy of variable - '__u' allocated on
+ * the stack.
*
* Their two major use cases are: (1) Mediating communication between
* process-level code and irq/NMI handlers, all running on the same CPU,
@@ -322,29 +322,4 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
compiletime_assert(__native_word(t), \
"Need native word sized stores/loads for atomicity.")
-/*
- * Prevent the compiler from merging or refetching accesses. The compiler
- * is also forbidden from reordering successive instances of ACCESS_ONCE(),
- * but only when the compiler is aware of some particular ordering. One way
- * to make the compiler aware of ordering is to put the two invocations of
- * ACCESS_ONCE() in different C statements.
- *
- * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
- * on a union member will work as long as the size of the member matches the
- * size of the union and the size is smaller than word size.
- *
- * The major use cases of ACCESS_ONCE used to be (1) Mediating communication
- * between process-level code and irq/NMI handlers, all running on the same CPU,
- * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
- * mutilate accesses that either do not require ordering or that interact
- * with an explicit memory barrier or atomic instruction that provides the
- * required ordering.
- *
- * If possible use READ_ONCE()/WRITE_ONCE() instead.
- */
-#define __ACCESS_ONCE(x) ({ \
- __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \
- (volatile typeof(x) *)&(x); })
-#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
-
#endif /* __LINUX_COMPILER_H */
diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 8b80bac055e4..fffe1e5895a2 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -6242,28 +6242,6 @@ 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($par)/;
- }
- }
- }
-
# check for mutex_trylock_recursive usage
if ($line =~ /mutex_trylock_recursive/) {
ERROR("LOCKING",
diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
index 07fd03c74a77..cb77706af769 100644
--- a/tools/include/linux/compiler.h
+++ b/tools/include/linux/compiler.h
@@ -84,8 +84,6 @@
#define uninitialized_var(x) x = *(&(x))
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
-
#include <linux/types.h>
/*
@@ -135,16 +133,15 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
/*
* Prevent the compiler from merging or refetching reads or writes. The
* compiler is also forbidden from reordering successive instances of
- * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
- * compiler is aware of some particular ordering. One way to make the
- * compiler aware of ordering is to put the two invocations of READ_ONCE,
- * WRITE_ONCE or ACCESS_ONCE() in different C statements.
+ * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
+ * particular ordering. One way to make the compiler aware of ordering is
+ * to put the two invocations of READ_ONCE or WRITE_ONCE in different C
+ * statements.
*
- * In contrast to ACCESS_ONCE these two macros will also work on aggregate
- * data types like structs or unions. If the size of the accessed data
- * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
- * READ_ONCE() and WRITE_ONCE() will fall back to memcpy and print a
- * compile-time warning.
+ * These two macros will also work on aggregate data types like structs or
+ * unions. If the size of the accessed data type exceeds the word size of the
+ * machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will fall
+ * back to memcpy and print a compile-time warning.
*
* Their two major use cases are: (1) Mediating communication between
* process-level code and irq/NMI handlers, all running on the same CPU,
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