linux-kernel - Re: [PATCH 12/13] locking/qspinlock: separate pv_wait

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Message-ID: <YsR2IQsnqAOgDxXu@worktop.programming.kicks-ass.net>
Date:   Tue, 5 Jul 2022 19:34:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 12/13] locking/qspinlock: separate pv_wait_node from the
 non-paravirt path

On Tue, Jul 05, 2022 at 12:38:19AM +1000, Nicholas Piggin wrote:
> pv_wait_node waits until node->locked is non-zero, no need for the
> pv case to wait again by also executing the !pv code path.
> 
> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
> ---
>  kernel/locking/qspinlock.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 9db168753124..19e2f286be0a 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -862,10 +862,11 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, bool parav
>  		/* Link @node into the waitqueue. */
>  		WRITE_ONCE(prev->next, node);
>  
> +		/* Wait for mcs node lock to be released */
>  		if (paravirt)
>  			pv_wait_node(node, prev);
> -		/* Wait for mcs node lock to be released */
> -		smp_cond_load_acquire(&node->locked, VAL);
> +		else
> +			smp_cond_load_acquire(&node->locked, VAL);
>  

(from patch #6):

+static void pv_wait_node(struct qnode *node, struct qnode *prev)
+{
+       int loop;
+       bool wait_early;
+
	...
+
+       /*
+        * By now our node->locked should be 1 and our caller will not actually
+        * spin-wait for it. We do however rely on our caller to do a
+        * load-acquire for us.
+        */
+}