Re: [PATCH v5 0/8] Add generic support for relaxed atomics

From: Boqun Feng
Date: Wed Aug 26 2015 - 00:28:59 EST

Hi Will,

On Thu, Aug 06, 2015 at 05:54:36PM +0100, Will Deacon wrote:
> Will Deacon (8):
> atomics: add acquire/release/relaxed variants of some atomic
> operations
> asm-generic: rework atomic-long.h to avoid bulk code duplication
> asm-generic: add relaxed/acquire/release variants for atomic_long_t
> lockref: remove homebrew cmpxchg64_relaxed macro definition
> locking/qrwlock: implement queue_write_unlock using smp_store_release
> locking/qrwlock: make use of acquire/release/relaxed atomics
> include/llist: use linux/atomic.h instead of asm/cmpxchg.h

Should we step further to privatize asm/cmpxchg.h entirely? Keep it only
included in arch/*?

Because after your next patch, in some configurations, asm/cmpxchg.h of
ARM only provides the definition of {cmpxchg,xchg}_relaxed, others are
built in linux/atomic.h. Further more, other architecture may implement
asm/cmpxchg.h similarly in the future. So, IIUC, we actually don't
guarantee all cmpxchg(), xchg() and their variants are defined in

Though current users of asm/cmpxchg.h outside arch/* are fine,
because they all happen to have got linux/atomic.h included. But we'd
better change the current users and call out that asm/cmpxchg.h is
privatized in the document.

> ARM: atomics: define our SMP atomics in terms of _relaxed operations

Consider this patch maybe? I did a simple build test on X86.


Subject: [PATCH] atomics,cmpxchg: Privatize the inclusion of asm/cmpxchg.h

After commit:

atomics: add acquire/release/relaxed variants of some atomic operations

Architectures may only provide {cmp,}xchg_relaxed definitions in
asm/cmpxchg.h. Other variants, such as {cmp,}xchg, may be built in
linux/atomic.h, which means simply including asm/cmpxchg.h may not get
the definitions of all the{cmp,}xchg variants. Therefore, we should
privatize the inclusions of asm/cmpxchg.h to keep it only included in
arch/* and replace the inclusions outside with linux/atomic.h

Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Documentation/atomic_ops.txt | 4 ++++
drivers/net/ethernet/sfc/mcdi.c | 2 +-
drivers/phy/phy-rcar-gen2.c | 3 +--
drivers/staging/speakup/selection.c | 2 +-
4 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
index b19fc34..c9d1cac 100644
--- a/Documentation/atomic_ops.txt
+++ b/Documentation/atomic_ops.txt
@@ -542,6 +542,10 @@ The routines xchg() and cmpxchg() must provide the same exact
memory-barrier semantics as the atomic and bit operations returning

+Note: If someone wants to use xchg(), cmpxchg() and their variants,
+linux/atomic.h should be included rather than asm/cmpxchg.h, unless
+the code is in arch/* and can take care of itself.
Spinlocks and rwlocks have memory barrier expectations as well.
The rule to follow is simple:

diff --git a/drivers/net/ethernet/sfc/mcdi.c b/drivers/net/ethernet/sfc/mcdi.c
index 81640f8..968383e 100644
--- a/drivers/net/ethernet/sfc/mcdi.c
+++ b/drivers/net/ethernet/sfc/mcdi.c
@@ -9,7 +9,7 @@

#include <linux/delay.h>
#include <linux/moduleparam.h>
-#include <asm/cmpxchg.h>
+#include <linux/atomic.h>
#include "net_driver.h"
#include "nic.h"
#include "io.h"
diff --git a/drivers/phy/phy-rcar-gen2.c b/drivers/phy/phy-rcar-gen2.c
index 39d9b29..117b495 100644
--- a/drivers/phy/phy-rcar-gen2.c
+++ b/drivers/phy/phy-rcar-gen2.c
@@ -17,8 +17,7 @@
#include <linux/phy/phy.h>
#include <linux/platform_device.h>
#include <linux/spinlock.h>
-#include <asm/cmpxchg.h>
+#include <linux/atomic.h>

#define USBHS_LPSTS 0x02
#define USBHS_UGCTRL 0x80
diff --git a/drivers/staging/speakup/selection.c b/drivers/staging/speakup/selection.c
index a031570..81c0888 100644
--- a/drivers/staging/speakup/selection.c
+++ b/drivers/staging/speakup/selection.c
@@ -7,7 +7,7 @@
#include <linux/workqueue.h>
#include <linux/tty.h>
#include <linux/tty_flip.h>
-#include <asm/cmpxchg.h>
+#include <linux/atomic.h>

#include "speakup.h"

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