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

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Message-ID: <20150826042834.GA10432@fixme-laptop.cn.ibm.com>
Date:	Wed, 26 Aug 2015 12:28:34 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-arch@...r.kernel.org, Waiman.Long@...com,
	peterz@...radead.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, mingo@...nel.org
Subject: Re: [PATCH v5 0/8] Add generic support for relaxed atomics

Hi Will,

On Thu, Aug 06, 2015 at 05:54:36PM +0100, Will Deacon wrote:
<snip>
> 
> 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
asm/cmpxchg.h.

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.

Regards,
Boqun

-----------------------------------------------------------------------
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@...il.com>
---
 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
 values.
 
+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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