Re: [Xen-devel] [PATCH v7 02/11] qspinlock, x86: Enable x86-64 to use queue spinlock

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Mar 19, 2014 at 04:14:00PM -0400, Waiman Long wrote:
> This patch makes the necessary changes at the x86 architecture
> specific layer to enable the use of queue spinlock for x86-64. As
> x86-32 machines are typically not multi-socket. The benefit of queue
> spinlock may not be apparent. So queue spinlock is not enabled.
> 
> Currently, there is some incompatibilities between the para-virtualized
> spinlock code (which hard-codes the use of ticket spinlock) and the
> queue spinlock. Therefore, the use of queue spinlock is disabled when
> the para-virtualized spinlock is enabled.

And how does this patch do that? I think that comment is obsolete?

> 
> The arch/x86/include/asm/qspinlock.h header file includes some x86
> specific optimization which will make the queue spinlock code perform
> better than the generic implementation.
> 
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
>  arch/x86/Kconfig                      |    1 +
>  arch/x86/include/asm/qspinlock.h      |   41 
> +++++++++++++++++++++++++++++++++
>  arch/x86/include/asm/spinlock.h       |    5 ++++
>  arch/x86/include/asm/spinlock_types.h |    4 +++
>  4 files changed, 51 insertions(+), 0 deletions(-)
>  create mode 100644 arch/x86/include/asm/qspinlock.h
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 0af5250..de573f9 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -17,6 +17,7 @@ config X86_64
>       depends on 64BIT
>       select X86_DEV_DMA_OPS
>       select ARCH_USE_CMPXCHG_LOCKREF
> +     select ARCH_USE_QUEUE_SPINLOCK
>  
>  ### Arch settings
>  config X86
> diff --git a/arch/x86/include/asm/qspinlock.h 
> b/arch/x86/include/asm/qspinlock.h
> new file mode 100644
> index 0000000..44cefee
> --- /dev/null
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -0,0 +1,41 @@
> +#ifndef _ASM_X86_QSPINLOCK_H
> +#define _ASM_X86_QSPINLOCK_H
> +
> +#include <asm-generic/qspinlock_types.h>
> +
> +#if !defined(CONFIG_X86_OOSTORE) && !defined(CONFIG_X86_PPRO_FENCE)
> +
> +#define _ARCH_SUPPORTS_ATOMIC_8_16_BITS_OPS
> +
> +/*
> + * x86-64 specific queue spinlock union structure
> + */
> +union arch_qspinlock {
> +     struct qspinlock slock;
> +     u8               lock;  /* Lock bit     */
> +};
> +
> +#define      queue_spin_unlock queue_spin_unlock
> +/**
> + * queue_spin_unlock - release a queue spinlock
> + * @lock : Pointer to queue spinlock structure
> + *
> + * No special memory barrier other than a compiler one is needed for the
> + * x86 architecture. A compiler barrier is added at the end to make sure
> + * that the clearing the lock bit is done ASAP without artificial delay
> + * due to compiler optimization.
> + */
> +static inline void queue_spin_unlock(struct qspinlock *lock)
> +{
> +     union arch_qspinlock *qlock = (union arch_qspinlock *)lock;
> +
> +     barrier();
> +     ACCESS_ONCE(qlock->lock) = 0;
> +     barrier();
> +}
> +
> +#endif /* !CONFIG_X86_OOSTORE && !CONFIG_X86_PPRO_FENCE */
> +
> +#include <asm-generic/qspinlock.h>
> +
> +#endif /* _ASM_X86_QSPINLOCK_H */
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index 0f62f54..958d20f 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -42,6 +42,10 @@
>  extern struct static_key paravirt_ticketlocks_enabled;
>  static __always_inline bool static_key_false(struct static_key *key);
>  
> +#ifdef CONFIG_QUEUE_SPINLOCK
> +#include <asm/qspinlock.h>
> +#else
> +
>  #ifdef CONFIG_PARAVIRT_SPINLOCKS
>  
>  static inline void __ticket_enter_slowpath(arch_spinlock_t *lock)
> @@ -180,6 +184,7 @@ static __always_inline void 
> arch_spin_lock_flags(arch_spinlock_t *lock,
>  {
>       arch_spin_lock(lock);
>  }
> +#endif /* CONFIG_QUEUE_SPINLOCK */
>  
>  static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
>  {
> diff --git a/arch/x86/include/asm/spinlock_types.h 
> b/arch/x86/include/asm/spinlock_types.h
> index 4f1bea1..7960268 100644
> --- a/arch/x86/include/asm/spinlock_types.h
> +++ b/arch/x86/include/asm/spinlock_types.h
> @@ -23,6 +23,9 @@ typedef u32 __ticketpair_t;
>  
>  #define TICKET_SHIFT (sizeof(__ticket_t) * 8)
>  
> +#ifdef CONFIG_QUEUE_SPINLOCK
> +#include <asm-generic/qspinlock_types.h>
> +#else
>  typedef struct arch_spinlock {
>       union {
>               __ticketpair_t head_tail;
> @@ -33,6 +36,7 @@ typedef struct arch_spinlock {
>  } arch_spinlock_t;
>  
>  #define __ARCH_SPIN_LOCK_UNLOCKED    { { 0 } }
> +#endif /* CONFIG_QUEUE_SPINLOCK */
>  
>  #include <asm/rwlock.h>
>  
> -- 
> 1.7.1
> 

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