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Date: Thu, 24 Oct 2013 13:58:35 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Waiman Long <Waiman.Long@...com>
Cc: Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
Rik van Riel <riel@...hat.com>,
Peter Hurley <peter@...leysoftware.com>,
Davidlohr Bueso <davidlohr.bueso@...com>,
Alex Shi <alex.shi@...aro.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Andrea Arcangeli <aarcange@...hat.com>,
Matthew R Wilcox <matthew.r.wilcox@...el.com>,
Dave Hansen <dave.hansen@...el.com>,
Michel Lespinasse <walken@...gle.com>,
Andi Kleen <andi@...stfloor.org>,
Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
George Spelvin <linux@...izon.com>,
"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
Aswin Chandramouleeswaran <aswin@...com>,
Scott J Norton <scott.norton@...com>,
Davidlohr Bueso <davidlohr@...com>,
Jason Low <jason.low2@...com>
Subject: Re: [PATCH v8 10/10] MCS Lock: Make mcs_spinlock.h includable in
other files
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@...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
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