Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <48a8bde8-8783-e1f6-ef44-c9cae15f71a7@redhat.com>
Date:   Tue, 5 Jul 2022 15:34:03 -0400
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.9.1
Subject: Re: [PATCH 04/13] locking/qspinlock: move pv lock word helpers into
 qspinlock.c
Content-Language: en-US
To:     Nicholas Piggin <npiggin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org
References: <20220704143820.3071004-1-npiggin@gmail.com>
 <20220704143820.3071004-5-npiggin@gmail.com>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20220704143820.3071004-5-npiggin@gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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@gmail.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.

Cheers,
Longman