Re: [PATCH] perf mmap: Convert ACCESS_ONCE() to READ_ONCE()

From: Paul E. McKenney
Date: Tue Nov 14 2017 - 17:56:21 EST

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On Tue, Nov 14, 2017 at 02:21:56PM -0800, Joe Perches wrote:
> 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@xxxxxxx>
> > > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >
> > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> >
> > Woo-hoo! Good to see that your Coccinelle script has already done
> > its magic! ;-)
>
> Might be nice to remove all the other references too

Agreed, and that is exactly what Mark was referring to in his "remove
the ACCESS_ONCE() definitions in subsequent patches". ;-)

Thanx, Paul

> $ 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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