linux-kernel - Re: [PATCH v2 04/13] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath

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Date:   Thu, 12 Apr 2018 15:18:27 +0100
From:   Will Deacon <will.deacon@....com>
To:     Waiman Long <longman@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        peterz@...radead.org, mingo@...nel.org, boqun.feng@...il.com,
        paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH v2 04/13] locking/qspinlock: Remove unbounded cmpxchg
 loop from locking slowpath

On Thu, Apr 12, 2018 at 10:16:55AM -0400, Waiman Long wrote:
> 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.

Yup, doing that now. I just botched my test scripts before, which default to
arm64.

Will