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    SubjectRe: [PATCH v2 04/13] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath
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    On 04/12/2018 10:06 AM, Will Deacon wrote:
    > On Wed, Apr 11, 2018 at 03:53:16PM -0400, Waiman Long wrote:
    >>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
    >>> index 396701e8c62d..a8fc402b3f3a 100644
    >>> --- a/kernel/locking/qspinlock.c
    >>> +++ b/kernel/locking/qspinlock.c
    >>> @@ -162,6 +162,17 @@ struct __qspinlock {
    >>>
    >>> #if _Q_PENDING_BITS == 8
    >>> /**
    >>> + * clear_pending - clear the pending bit.
    >>> + * @lock: Pointer to queued spinlock structure
    >>> + *
    >>> + * *,1,* -> *,0,*
    >>> + */
    >>> +static __always_inline void clear_pending(struct qspinlock *lock)
    >>> +{
    >>> + WRITE_ONCE(lock->pending, 0);
    >>> +}
    >>> +
    >>> +/**
    >>> * clear_pending_set_locked - take ownership and clear the pending bit.
    >>> * @lock: Pointer to queued spinlock structure
    >>> *
    >>> @@ -201,6 +212,17 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
    >>> #else /* _Q_PENDING_BITS == 8 */
    >>>
    >>> /**
    >>> + * clear_pending - clear the pending bit.
    >>> + * @lock: Pointer to queued spinlock structure
    >>> + *
    >>> + * *,1,* -> *,0,*
    >>> + */
    >>> +static __always_inline void clear_pending(struct qspinlock *lock)
    >>> +{
    >>> + atomic_andnot(_Q_PENDING_VAL, &lock->val);
    >>> +}
    >>> +
    >>> +/**
    >>> * clear_pending_set_locked - take ownership and clear the pending bit.
    >>> * @lock: Pointer to queued spinlock structure
    >>> *
    >> BTW, there is a similar clear_pending() function in
    >> qspinlock_paravirt.c. I think you need to remove that with this patch.
    > Thanks, I'll do that. I did build and bisect this series... for arm64, which
    > is completely useless as it doesn't get compiled until the final patch which
    > I haven't posted here.
    >
    > Will

    You certainly need to get a x86 system and compiles with the patches
    applied one-by-one with CONFIG_PARAVIRT_SPINLOCKS enabled.

    Cheers,
    Longman

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