[PATCH v8 10/10] MCS Lock: Make mcs_spinlock.h includable in other files

[PATCH v8 10/10] MCS Lock: Make mcs_spinlock.h includable in other files

[Waiman Long]

The following changes are made to enable mcs_spinlock.h file to be
widely included in other files without causing problem:

1) Include a number of prerequisite header files and define
   arch_mutex_cpu_relax(), if not previously defined.
2) Separate out mcs_spin_lock() into a mcs_spinlock.c file.
3) Make mcs_spin_unlock() an inlined function.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
---
 include/linux/mcs_spinlock.h |   43 ++++++++++++++++-------------------------
 kernel/Makefile              |    6 ++--
 kernel/mcs_spinlock.c        |   37 ++++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+), 29 deletions(-)
 create mode 100644 kernel/mcs_spinlock.c

diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
index b5de3b0..62979f3 100644
--- a/include/linux/mcs_spinlock.h
+++ b/include/linux/mcs_spinlock.h
@@ -12,38 +12,29 @@
 #ifndef __LINUX_MCS_SPINLOCK_H
 #define __LINUX_MCS_SPINLOCK_H
 
+/*
+ * 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>
+
+#ifndef arch_mutex_cpu_relax
+# define arch_mutex_cpu_relax() cpu_relax()
+#endif
+
 struct mcs_spinlock {
 	struct mcs_spinlock *next;
 	int locked; /* 1 if lock acquired */
 };
 
-/*
- * We don't inline mcs_spin_lock() so that perf can correctly account for the
- * time spent in this lock function.
- */
-static noinline
-void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
-{
-	struct mcs_spinlock *prev;
-
-	/* Init node */
-	node->locked = 0;
-	node->next   = NULL;
-
-	prev = xchg(lock, node);
-	if (likely(prev == NULL)) {
-		/* Lock acquired */
-		node->locked = 1;
-		return;
-	}
-	ACCESS_ONCE(prev->next) = node;
-	smp_wmb();
-	/* Wait until the lock holder passes the lock down */
-	while (!ACCESS_ONCE(node->locked))
-		arch_mutex_cpu_relax();
-}
+extern
+void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node);
 
-static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
+static inline
+void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 {
 	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
 
diff --git a/kernel/Makefile b/kernel/Makefile
index 1ce4755..2ad8454 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -50,9 +50,9 @@ obj-$(CONFIG_SMP) += smp.o
 ifneq ($(CONFIG_SMP),y)
 obj-y += up.o
 endif
-obj-$(CONFIG_SMP) += spinlock.o
-obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
-obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
+obj-$(CONFIG_SMP) += spinlock.o mcs_spinlock.o
+obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o mcs_spinlock.o
+obj-$(CONFIG_PROVE_LOCKING) += spinlock.o mcs_spinlock.o
 obj-$(CONFIG_UID16) += uid16.o
 obj-$(CONFIG_MODULES) += module.o
 obj-$(CONFIG_MODULE_SIG) += module_signing.o modsign_pubkey.o modsign_certificate.o
diff --git a/kernel/mcs_spinlock.c b/kernel/mcs_spinlock.c
new file mode 100644
index 0000000..6b20324
--- /dev/null
+++ b/kernel/mcs_spinlock.c
@@ -0,0 +1,37 @@
+/*
+ * MCS lock
+ *
+ * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
+ * with the desirable properties of being fair, and with each cpu trying
+ * to acquire the lock spinning on a local variable.
+ * It avoids expensive cache bouncings that common test-and-set spin-lock
+ * implementations incur.
+ */
+#include <linux/mcs_spinlock.h>
+#include <linux/export.h>
+
+/*
+ * We don't inline mcs_spin_lock() so that perf can correctly account for the
+ * time spent in this lock function.
+ */
+void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
+{
+	struct mcs_spinlock *prev;
+
+	/* Init node */
+	node->locked = 0;
+	node->next   = NULL;
+
+	prev = xchg(lock, node);
+	if (likely(prev == NULL)) {
+		/* Lock acquired */
+		node->locked = 1;
+		return;
+	}
+	ACCESS_ONCE(prev->next) = node;
+	smp_wmb();
+	/* Wait until the lock holder passes the lock down */
+	while (!ACCESS_ONCE(node->locked))
+		arch_mutex_cpu_relax();
+}
+EXPORT_SYMBOL(mcs_spin_lock);
-- 
1.7.1

Re: [PATCH v8 10/10] MCS Lock: Make mcs_spinlock.h includable in other files

[Tim Chen]

On Thu, 2013-10-24 at 16:01 -0400, Waiman Long wrote:
> The following changes are made to enable mcs_spinlock.h file to be
> widely included in other files without causing problem:
> 
> 1) Include a number of prerequisite header files and define
>    arch_mutex_cpu_relax(), if not previously defined.
> 2) Separate out mcs_spin_lock() into a mcs_spinlock.c file.
> 3) Make mcs_spin_unlock() an inlined function.
> 
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>
> ---
>  include/linux/mcs_spinlock.h |   43 ++++++++++++++++-------------------------
>  kernel/Makefile              |    6 ++--
>  kernel/mcs_spinlock.c        |   37 ++++++++++++++++++++++++++++++++++++
>  3 files changed, 57 insertions(+), 29 deletions(-)
>  create mode 100644 kernel/mcs_spinlock.c
> 
> diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
> index b5de3b0..62979f3 100644
> --- a/include/linux/mcs_spinlock.h
> +++ b/include/linux/mcs_spinlock.h
> @@ -12,38 +12,29 @@
>  #ifndef __LINUX_MCS_SPINLOCK_H
>  #define __LINUX_MCS_SPINLOCK_H
>  
> +/*
> + * 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>
> +
> +#ifndef arch_mutex_cpu_relax
> +# define arch_mutex_cpu_relax() cpu_relax()
> +#endif
> +
>  struct mcs_spinlock {
>  	struct mcs_spinlock *next;
>  	int locked; /* 1 if lock acquired */
>  };
>  
> -/*
> - * We don't inline mcs_spin_lock() so that perf can correctly account for the
> - * time spent in this lock function.
> - */
> -static noinline
> -void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> -{
> -	struct mcs_spinlock *prev;
> -
> -	/* Init node */
> -	node->locked = 0;
> -	node->next   = NULL;
> -
> -	prev = xchg(lock, node);
> -	if (likely(prev == NULL)) {
> -		/* Lock acquired */
> -		node->locked = 1;
> -		return;
> -	}
> -	ACCESS_ONCE(prev->next) = node;
> -	smp_wmb();
> -	/* Wait until the lock holder passes the lock down */
> -	while (!ACCESS_ONCE(node->locked))
> -		arch_mutex_cpu_relax();
> -}
> +extern
> +void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node);
>  
> -static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> +static inline
> +void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  {
>  	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
>  

Do we want to inline the unlock?  Will that prevent proper profile 
accounting of unlock overhead?  

Can we keep the mcs_spin_unlock and mcs_spin_lock in the same
kernel/mcs_spinlock.c file? That makes it easier to read and 
maintain the code.

> diff --git a/kernel/Makefile b/kernel/Makefile
> index 1ce4755..2ad8454 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -50,9 +50,9 @@ obj-$(CONFIG_SMP) += smp.o
>  ifneq ($(CONFIG_SMP),y)
>  obj-y += up.o
>  endif
> -obj-$(CONFIG_SMP) += spinlock.o
> -obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
> -obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
> +obj-$(CONFIG_SMP) += spinlock.o mcs_spinlock.o
> +obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o mcs_spinlock.o
> +obj-$(CONFIG_PROVE_LOCKING) += spinlock.o mcs_spinlock.o
>  obj-$(CONFIG_UID16) += uid16.o
>  obj-$(CONFIG_MODULES) += module.o
>  obj-$(CONFIG_MODULE_SIG) += module_signing.o modsign_pubkey.o modsign_certificate.o
> diff --git a/kernel/mcs_spinlock.c b/kernel/mcs_spinlock.c
> new file mode 100644
> index 0000000..6b20324
> --- /dev/null
> +++ b/kernel/mcs_spinlock.c
> @@ -0,0 +1,37 @@
> +/*
> + * MCS lock
> + *
> + * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
> + * with the desirable properties of being fair, and with each cpu trying
> + * to acquire the lock spinning on a local variable.
> + * It avoids expensive cache bouncings that common test-and-set spin-lock
> + * implementations incur.
> + */
> +#include <linux/mcs_spinlock.h>
> +#include <linux/export.h>
> +
> +/*
> + * We don't inline mcs_spin_lock() so that perf can correctly account for the
> + * time spent in this lock function.
> + */
> +void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> +{
> +	struct mcs_spinlock *prev;
> +
> +	/* Init node */
> +	node->locked = 0;
> +	node->next   = NULL;
> +
> +	prev = xchg(lock, node);
> +	if (likely(prev == NULL)) {
> +		/* Lock acquired */
> +		node->locked = 1;
> +		return;
> +	}
> +	ACCESS_ONCE(prev->next) = node;
> +	smp_wmb();
> +	/* Wait until the lock holder passes the lock down */
> +	while (!ACCESS_ONCE(node->locked))
> +		arch_mutex_cpu_relax();
> +}
> +EXPORT_SYMBOL(mcs_spin_lock);

Can you check if you have applied all the previous MCS patches?
The last two for barrier corrections and optimizations seem
to be missing.

MCS Lock: optimizations and extra comments
https://lkml.org/lkml/2013/10/2/644
MCS Lock: Barrier corrections
https://lkml.org/lkml/2013/10/2/650

Thanks.

Tim

Re: [PATCH v8 10/10] MCS Lock: Make mcs_spinlock.h includable in other files

[Waiman Long]

On 10/24/2013 04:58 PM, Tim Chen wrote:
>
> Do we want to inline the unlock?  Will that prevent proper profile
> accounting of unlock overhead?
>
> Can we keep the mcs_spin_unlock and mcs_spin_lock in the same
> kernel/mcs_spinlock.c file? That makes it easier to read and
> maintain the code.

The unlock code is fast. The lock code, however, can run for a long 
time. It will greatly increase the reported time spent in the calling 
function if it is inlined. The same is true for spinlock. The 
_raw_spin_lock() is a real function while _raw_spin_unlock() is inlined 
in most cases.

Yes, I can bring the lock function back to the mcs_spinlock.h file with 
name like _raw_mcs_spin_lock() and the mcs_spin_lock() in mcs_spinlock.c 
will include the raw function. In that way, the mcs_spin_lock() will 
still be a separate function while both the lock and unlock code will be 
together.

> Can you check if you have applied all the previous MCS patches?
> The last two for barrier corrections and optimizations seem
> to be missing.
>
> MCS Lock: optimizations and extra comments
> https://lkml.org/lkml/2013/10/2/644
> MCS Lock: Barrier corrections
> https://lkml.org/lkml/2013/10/2/650
>
> Thanks.
>
> Tim
>

Apparently, I does have all the MCS patch  in my git tree. I will 
regenerate a new one with the right diff. Thank for the review.

-Longman