[Xen-devel] Re: [PATCH 02/20] x86/ticketlock: convert spin loop to C

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

Le mercredi 03 novembre 2010 à 10:59 -0400, Jeremy Fitzhardinge a
écrit :
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
> 
> The inner loop of __ticket_spin_lock isn't doing anything very special,
> so reimplement it in C.
> 
> For the 8 bit ticket lock variant, we use a register union to get direct
> access to the lower and upper bytes in the tickets, but unfortunately gcc
> won't generate a direct comparison between the two halves of the register,
> so the generated asm isn't quite as pretty as the hand-coded version.
> However benchmarking shows that this is actually a small improvement in
> runtime performance on some benchmarks, and never a slowdown.
> 
> We also need to make sure there's a barrier at the end of the lock loop
> to make sure that the compiler doesn't move any instructions from within
> the locked region into the region where we don't yet own the lock.
> 
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/spinlock.h |   58 +++++++++++++++++++-------------------
>  1 files changed, 29 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index d6d5784..6711d36 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -58,21 +58,21 @@
>  #if (NR_CPUS < 256)
>  static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>  {
> -     unsigned short inc = 1 << TICKET_SHIFT;
> -
> -     asm volatile (
> -             LOCK_PREFIX "xaddw %w0, %1\n"
> -             "1:\t"
> -             "cmpb %h0, %b0\n\t"
> -             "je 2f\n\t"
> -             "rep ; nop\n\t"
> -             "movb %1, %b0\n\t"
> -             /* don't need lfence here, because loads are in-order */
> -             "jmp 1b\n"
> -             "2:"
> -             : "+Q" (inc), "+m" (lock->slock)
> -             :
> -             : "memory", "cc");
> +     register union {
> +             struct __raw_tickets tickets;
> +             unsigned short slock;
> +     } inc = { .slock = 1 << TICKET_SHIFT };
> +
> +     asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
> +                   : "+Q" (inc), "+m" (lock->slock) : : "memory", "cc");
> +
> +     for (;;) {
> +             if (inc.tickets.head == inc.tickets.tail)
> +                     return;
> +             cpu_relax();
> +             inc.tickets.head = ACCESS_ONCE(lock->tickets.head);
> +     }
> +     barrier();              /* make sure nothing creeps before the lock is 
> taken */

Isnt this barrier() never reached ?


>  }
>  
>  static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
> @@ -105,22 +105,22 @@ static __always_inline void 
> __ticket_spin_unlock(arch_spinlock_t *lock)
>  static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>  {
>       unsigned inc = 1 << TICKET_SHIFT;
> -     unsigned tmp;
> +     __ticket_t tmp;
>  
> -     asm volatile(LOCK_PREFIX "xaddl %0, %1\n"
> -                  "movzwl %w0, %2\n\t"
> -                  "shrl $16, %0\n\t"
> -                  "1:\t"
> -                  "cmpl %0, %2\n\t"
> -                  "je 2f\n\t"
> -                  "rep ; nop\n\t"
> -                  "movzwl %1, %2\n\t"
> -                  /* don't need lfence here, because loads are in-order */
> -                  "jmp 1b\n"
> -                  "2:"
> -                  : "+r" (inc), "+m" (lock->slock), "=&r" (tmp)
> -                  :
> -                  : "memory", "cc");
> +     asm volatile(LOCK_PREFIX "xaddl %0, %1\n\t"
> +                  : "+r" (inc), "+m" (lock->slock)
> +                  : : "memory", "cc");
> +
> +     tmp = inc;
> +     inc >>= TICKET_SHIFT;
> +
> +     for (;;) {
> +             if ((__ticket_t)inc == tmp)
> +                     return;
> +             cpu_relax();
> +             tmp = ACCESS_ONCE(lock->tickets.head);
> +     }
> +     barrier();              /* make sure nothing creeps before the lock is 
> taken */

same here

>  }
>  
>  static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)



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