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    Subject[PATCH RFC V7 8/12] x86/pvticketlock: when paravirtualizing ticket locks, increment by 2
    From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

    Increment ticket head/tails by 2 rather than 1 to leave the LSB free
    to store a "is in slowpath state" bit. This halves the number
    of possible CPUs for a given ticket size, but this shouldn't matter
    in practice - kernels built for 32k+ CPU systems are probably
    specially built for the hardware rather than a generic distro
    kernel.

    Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
    Tested-by: Attilio Rao <attilio.rao@citrix.com>
    Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
    ---
    arch/x86/include/asm/spinlock.h | 10 +++++-----
    arch/x86/include/asm/spinlock_types.h | 10 +++++++++-
    2 files changed, 14 insertions(+), 6 deletions(-)
    diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
    index ee4bbd4..60b7e83 100644
    --- a/arch/x86/include/asm/spinlock.h
    +++ b/arch/x86/include/asm/spinlock.h
    @@ -81,7 +81,7 @@ static __always_inline void __ticket_unlock_kick(struct arch_spinlock *lock,
    */
    static __always_inline void arch_spin_lock(struct arch_spinlock *lock)
    {
    - register struct __raw_tickets inc = { .tail = 1 };
    + register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };

    inc = xadd(&lock->tickets, inc);

    @@ -107,7 +107,7 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
    if (old.tickets.head != old.tickets.tail)
    return 0;

    - new.head_tail = old.head_tail + (1 << TICKET_SHIFT);
    + new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT);

    /* cmpxchg is a full barrier, so nothing can move before it */
    return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == old.head_tail;
    @@ -115,9 +115,9 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)

    static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
    {
    - __ticket_t next = lock->tickets.head + 1;
    + __ticket_t next = lock->tickets.head + TICKET_LOCK_INC;

    - __add(&lock->tickets.head, 1, UNLOCK_LOCK_PREFIX);
    + __add(&lock->tickets.head, TICKET_LOCK_INC, UNLOCK_LOCK_PREFIX);
    __ticket_unlock_kick(lock, next);
    }

    @@ -132,7 +132,7 @@ static inline int arch_spin_is_contended(arch_spinlock_t *lock)
    {
    struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);

    - return (__ticket_t)(tmp.tail - tmp.head) > 1;
    + return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;
    }
    #define arch_spin_is_contended arch_spin_is_contended

    diff --git a/arch/x86/include/asm/spinlock_types.h b/arch/x86/include/asm/spinlock_types.h
    index 83fd3c7..e96fcbd 100644
    --- a/arch/x86/include/asm/spinlock_types.h
    +++ b/arch/x86/include/asm/spinlock_types.h
    @@ -3,7 +3,13 @@

    #include <linux/types.h>

    -#if (CONFIG_NR_CPUS < 256)
    +#ifdef CONFIG_PARAVIRT_SPINLOCKS
    +#define __TICKET_LOCK_INC 2
    +#else
    +#define __TICKET_LOCK_INC 1
    +#endif
    +
    +#if (CONFIG_NR_CPUS < (256 / __TICKET_LOCK_INC))
    typedef u8 __ticket_t;
    typedef u16 __ticketpair_t;
    #else
    @@ -11,6 +17,8 @@ typedef u16 __ticket_t;
    typedef u32 __ticketpair_t;
    #endif

    +#define TICKET_LOCK_INC ((__ticket_t)__TICKET_LOCK_INC)
    +
    #define TICKET_SHIFT (sizeof(__ticket_t) * 8)

    typedef struct arch_spinlock {


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