linux-kernel - Re: [PATCH 04/13] locking/qspinlock: move pv lock word helpers into qspinlock.c

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Date:   Tue, 12 Jul 2022 10:11:21 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH 04/13] locking/qspinlock: move pv lock word helpers into
 qspinlock.c

Excerpts from Waiman Long's message of July 6, 2022 5:34 am:
> On 7/4/22 10:38, Nicholas Piggin wrote:
>> There is no real reason not to keep all the bit manipulation together.
>>
>> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
>> ---
>>   kernel/locking/qspinlock.c          | 107 ++++++++++++++++------------
>>   kernel/locking/qspinlock_paravirt.h |  51 -------------
>>   2 files changed, 63 insertions(+), 95 deletions(-)
>>
>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>> index 7360d643de29..8f2173e22479 100644
>> --- a/kernel/locking/qspinlock.c
>> +++ b/kernel/locking/qspinlock.c
>> @@ -141,7 +141,24 @@ struct qnode *grab_qnode(struct qnode *base, int idx)
>>   
>>   #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
>>   
>> +/**
>> + * set_pending - set the pending bit.
>> + * @lock: Pointer to queued spinlock structure
>> + *
>> + * *,0,* -> *,1,*
>> + *
>> + * The pending bit is used by the queue head vCPU to indicate that it
>> + * is actively spinning on the lock and no lock stealing is allowed.
> 
> The pending bit has different usage in pv and non-pv cases. The 
> description here refers to the pv case. For non-pv, it is used to avoid 
> loading the the extra node cacheline in likely contended case.

Ah thank you, I can expand on that.

Thanks,
Nick