On Wed, Nov 20, 2013 at 12:50:23PM +0000, Paul E. McKenney wrote:
> On Wed, Nov 20, 2013 at 10:19:57AM +0000, Will Deacon wrote:
> > On Wed, Nov 20, 2013 at 01:37:26AM +0000, Tim Chen wrote:
> > > Will, do you want to take a crack at adding implementation for ARM
> > > with wfe instruction?
> >
> > Sure, I'll have a go this week. Thanks for keeping that as a consideration!
> >
> > As an aside: what are you using to test this code, so that I can make sure I
> > don't break it?
>
> +1 to that! In fact, it would be nice to have the test code in-tree,
> especially if it can test a wide variety of locks. (/me needs to look
> at what test code for locks might already be in tree, for that matter...)
Well, in the absence of those tests, I've implemented something that I think
will work for ARM and could be easily extended to arm64.
Tim: I reverted your final patch and went with Paul's suggestion just to
look into the contended case. I'm also not sure about adding
asm/mcs_spinlock.h. This stuff might be better in asm/spinlock.h, which
already exists and contains both spinlocks and rwlocks. Depends on how much
people dislike the Kconfig symbol + conditional #include.
Anyway, patches below. I included the ARM bits for reference, but please
don't include them in your series!
Cheers,
Will
--->8
>From 074f4cdf9ddc97454467b9ad9f85128ee67c5604 Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon@xxxxxxx>
Date: Wed, 20 Nov 2013 16:14:04 +0000
Subject: [PATCH 1/3] MCS Lock: allow architectures to hook in to contended
paths
When contended, architectures may be able to reduce the polling overhead
in ways which aren't expressible using a simple relax() primitive.
This patch allows architectures to hook into the mcs_{lock,unlock}
functions for the contended cases only.
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
kernel/locking/mcs_spinlock.c | 47 +++++++++++++++++++++++++------------------
1 file changed, 27 insertions(+), 20 deletions(-)
diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
index 6f2ce8efb006..853070b8a86d 100644
--- a/kernel/locking/mcs_spinlock.c
+++ b/kernel/locking/mcs_spinlock.c
@@ -7,19 +7,34 @@
* It avoids expensive cache bouncings that common test-and-set spin-lock
* implementations incur.
*/
-/*
- * asm/processor.h may define arch_mutex_cpu_relax().
- * If it is not defined, cpu_relax() will be used.
- */
+
#include <asm/barrier.h>
#include <asm/cmpxchg.h>
#include <asm/processor.h>
#include <linux/compiler.h>
#include <linux/mcs_spinlock.h>
+#include <linux/mutex.h>
#include <linux/export.h>
-#ifndef arch_mutex_cpu_relax
-# define arch_mutex_cpu_relax() cpu_relax()
+#ifndef arch_mcs_spin_lock_contended
+/*
+ * Using smp_load_acquire() provides a memory barrier that ensures
+ * subsequent operations happen after the lock is acquired.
+ */
+#define arch_mcs_spin_lock_contended(l) \
+ while (!(smp_load_acquire(l))) { \
+ arch_mutex_cpu_relax(); \
+ }
+#endif
+
+#ifndef arch_mcs_spin_unlock_contended
+/*
+ * smp_store_release() provides a memory barrier to ensure all
+ * operations in the critical section has been completed before
+ * unlocking.
+ */
+#define arch_mcs_spin_unlock_contended(l) \
+ smp_store_release((l), 1)
#endif
/*
@@ -44,13 +59,9 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
return;
}
ACCESS_ONCE(prev->next) = node;
- /*
- * Wait until the lock holder passes the lock down.
- * Using smp_load_acquire() provides a memory barrier that
- * ensures subsequent operations happen after the lock is acquired.
- */
- while (!(smp_load_acquire(&node->locked)))
- arch_mutex_cpu_relax();
+
+ /* Wait until the lock holder passes the lock down. */
+ arch_mcs_spin_lock_contended(&node->locked);
}
EXPORT_SYMBOL_GPL(mcs_spin_lock);
@@ -72,12 +83,8 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
while (!(next = ACCESS_ONCE(node->next)))
arch_mutex_cpu_relax();
}
- /*
- * Pass lock to next waiter.
- * smp_store_release() provides a memory barrier to ensure
- * all operations in the critical section has been completed
- * before unlocking.
- */
- smp_store_release(&next->locked, 1);
+
+ /* Pass lock to next waiter. */
+ arch_mcs_spin_unlock_contended(&next->locked);
}
EXPORT_SYMBOL_GPL(mcs_spin_unlock);
--
1.8.2.2
>From faa48f77a17cfd99562b1e36de278367aa4d389c Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon@xxxxxxx>
Date: Wed, 20 Nov 2013 16:10:57 +0000
Subject: [PATCH 2/3] MCS Lock: add Kconfig entries to allow arch-specific
hooks
This patch adds Kconfig entries to allow architectures to hook into the
MCS lock/unlock functions in the contended case.
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
arch/Kconfig | 3 +++
include/linux/mcs_spinlock.h | 8 ++++++++
2 files changed, 11 insertions(+)
diff --git a/arch/Kconfig b/arch/Kconfig
index f1cf895c040f..ae738f706325 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -303,6 +303,9 @@ config HAVE_CMPXCHG_LOCAL
config HAVE_CMPXCHG_DOUBLE
bool
+config HAVE_ARCH_MCS_LOCK
+ bool
+
config ARCH_WANT_IPC_PARSE_VERSION
bool
diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
index d54bb232a238..d2c02adb0bbd 100644
--- a/include/linux/mcs_spinlock.h
+++ b/include/linux/mcs_spinlock.h
@@ -12,6 +12,14 @@
#ifndef __LINUX_MCS_SPINLOCK_H
#define __LINUX_MCS_SPINLOCK_H
+/*
+ * An architecture may provide its own lock/unlock functions for the
+ * contended case.
+ */
+#ifdef CONFIG_HAVE_ARCH_MCS_LOCK
+#include <asm/mcs_spinlock.h>
+#endif
+
struct mcs_spinlock {
struct mcs_spinlock *next;
int locked; /* 1 if lock acquired */
--
1.8.2.2
>From 21f047d40002ec4f1b780eee88f16a1870ab00ef Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon@xxxxxxx>
Date: Wed, 20 Nov 2013 16:15:31 +0000
Subject: [PATCH 3/3] ARM: mcs lock: implement wfe-based polling for MCS
locking
This patch introduces a wfe-based polling loop for spinning on contended
MCS locks and waking up corresponding waiters when the lock is released.
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
arch/arm/Kconfig | 1 +
arch/arm/include/asm/mcs_spinlock.h | 20 ++++++++++++++++++++
2 files changed, 21 insertions(+)
create mode 100644 arch/arm/include/asm/mcs_spinlock.h
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 214b698cefea..ab9fb84599ac 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -25,6 +25,7 @@ config ARM
select HARDIRQS_SW_RESEND
select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
select HAVE_ARCH_KGDB
+ select HAVE_ARCH_MCS_LOCK
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_TRACEHOOK
select HAVE_BPF_JIT
diff --git a/arch/arm/include/asm/mcs_spinlock.h b/arch/arm/include/asm/mcs_spinlock.h
new file mode 100644
index 000000000000..f32f97e81471
--- /dev/null
+++ b/arch/arm/include/asm/mcs_spinlock.h
@@ -0,0 +1,20 @@
+#ifndef __ASM_MCS_LOCK_H
+#define __ASM_MCS_LOCK_H
+
+/* MCS spin-locking. */
+#define arch_mcs_spin_lock_contended(lock) \
+do { \
+ /* Ensure prior stores are observed before we enter wfe. */ \
+ smp_mb(); \
+ while (!(smp_load_acquire(lock))) \
+ wfe(); \
+} while (0) \
+
+#define arch_mcs_spin_unlock_contended(lock) \
+do { \
+ smp_store_release(lock, 1); \
+ dsb(ishst); \
+ sev(); \
+} while (0)
+
+#endif /* __ASM_MCS_LOCK_H */
--
1.8.2.2
--
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