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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Will Deacon <will@kernel.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Android Kernel Team <kernel-team@android.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Peter Zijlstra <peterz@infradead.org>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: [RFC PATCH 6/8] READ_ONCE: Drop pointer qualifiers when reading from scalar types
Date: Fri, 10 Jan 2020 10:54:27 -0800
Message-ID: <CAHk-=wia5ppBsfHLMx648utCjO01JAZiME0K0eSHmhWuRyL+6w@mail.gmail.com> (raw)
In-Reply-To: <20200110165636.28035-7-will@kernel.org>

[-- Attachment #1: Type: text/plain, Size: 643 bytes --]

On Fri, Jan 10, 2020 at 8:56 AM Will Deacon <will@kernel.org> wrote:
>
> +/* Declare an unqualified scalar type. Leaves non-scalar types unchanged. */
> +#define __unqual_scalar_typeof(x) typeof(                                      \

Ugh. My eyes. That's horrendous.

I can't see any better alternatives, but it does make me go "Eww".

Well, I do see one possible alternative: just re-write the bitop
implementations in terms of "atomic_long_t", and just avoid the issue
entirely.

IOW, do something like the attached (but fleshed out and tested - this
patch has not seen a compiler, much less any thought at all).

                   Linus

[-- Attachment #2: patch.diff --]
[-- Type: text/x-patch, Size: 1107 bytes --]

 include/asm-generic/bitops/lock.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
index 3ae021368f48..071d8bfd86e5 100644
--- a/include/asm-generic/bitops/lock.h
+++ b/include/asm-generic/bitops/lock.h
@@ -6,6 +6,12 @@
 #include <linux/compiler.h>
 #include <asm/barrier.h>
 
+/* Drop the volatile, we will be doing READ_ONCE by hand */
+static inline atomic_long_t *atomic_long_bit_word(unsigned int nr, volatile unsigned long *p)
+{
+	return BIT_WORD(nr) + (atomic_long_t *)p;
+}
+
 /**
  * test_and_set_bit_lock - Set a bit and return its old value, for lock
  * @nr: Bit to set
@@ -20,12 +26,12 @@ static inline int test_and_set_bit_lock(unsigned int nr,
 {
 	long old;
 	unsigned long mask = BIT_MASK(nr);
+	atomic_long_t *loc = atomic_long_bit_word(nr, p);
 
-	p += BIT_WORD(nr);
-	if (READ_ONCE(*p) & mask)
+	if (atomic_read(loc) & mask)
 		return 1;
 
-	old = atomic_long_fetch_or_acquire(mask, (atomic_long_t *)p);
+	old = atomic_long_fetch_or_acquire(mask, loc);
 	return !!(old & mask);
 }
 

  reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-10 16:56 [RFC PATCH 0/8] Rework READ_ONCE() to improve codegen Will Deacon
2020-01-10 16:56 ` [RFC PATCH 1/8] compiler/gcc: Emit build-time warning for GCC prior to version 4.8 Will Deacon
2020-01-10 17:35   ` Arnd Bergmann
2020-01-10 17:53     ` Joe Perches
2020-01-13 14:39       ` Arnd Bergmann
2020-01-13 15:35         ` Masahiro Yamada
2020-01-13 14:27     ` Will Deacon
2020-01-14 21:39     ` Nick Desaulniers
2020-01-15 10:35       ` David Laight
2020-01-15 10:49       ` Arnd Bergmann
2020-01-10 16:56 ` [RFC PATCH 2/8] netfilter: Avoid assigning 'const' pointer to non-const pointer Will Deacon
2020-01-10 16:56 ` [RFC PATCH 3/8] fault_inject: Don't rely on "return value" from WRITE_ONCE() Will Deacon
2020-01-10 16:56 ` [RFC PATCH 4/8] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() Will Deacon
2020-01-10 16:56 ` [RFC PATCH 5/8] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses Will Deacon
2020-01-10 19:24   ` Arnd Bergmann
2020-01-13 16:16     ` Will Deacon
2020-01-10 16:56 ` [RFC PATCH 6/8] READ_ONCE: Drop pointer qualifiers when reading from scalar types Will Deacon
2020-01-10 18:54   ` Linus Torvalds [this message]
2020-01-13 14:59     ` Will Deacon
2020-01-13 17:42       ` Luc Van Oostenryck
2020-01-13 19:31       ` Linus Torvalds
2020-01-10 16:56 ` [RFC PATCH 7/8] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros Will Deacon
2020-01-10 19:42   ` Arnd Bergmann
2020-01-13 15:01     ` Will Deacon
2020-01-10 16:56 ` [RFC PATCH 8/8] arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros Will Deacon
2020-01-10 17:45   ` Mark Rutland
2020-01-10 17:58 ` [RFC PATCH 0/8] Rework READ_ONCE() to improve codegen Arnd Bergmann
2020-01-10 19:46 ` Arnd Bergmann
2020-01-10 20:14   ` Linus Torvalds
2020-01-13 13:03     ` Arnd Bergmann
2020-01-13 11:20 ` David Laight
2020-01-13 12:40 ` Christian Borntraeger

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