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 10:16:55 -0400
From:   Waiman Long <longman@...hat.com>
To:     Will Deacon <will.deacon@....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 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