linux-kernel - Re: [PATCH 2/3] srcu: queue work without holding the lock

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Date:   Thu, 12 Oct 2017 10:53:35 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] srcu: queue work without holding the lock

On 2017-10-10 14:43:13 [-0700], Paul E. McKenney wrote:
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 6d5880089ff6..558f9e7b283e 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -830,7 +866,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
>  	rhp->func = func;
>  	local_irq_save(flags);
>  	sdp = this_cpu_ptr(sp->sda);
> -	raw_spin_lock_rcu_node(sdp);
> +	spin_lock_rcu_node(sdp);

This and the same thing in srcu_might_be_idle() does not work because
  local_irq_save() + spin_lock() != spin_lock_irqsave()
but
  local_irq_save() + raw_spinlock = raw_spin_lock_irqsave()

I think that preempt_disable() for a stable this_cpu_ptr() is enough
here. I replaced local_irq_save() with local_lock_irqsave() on RT which
provides a per-CPU spinlock (for mutual exclusion) and disables
interrupts in !RT mode.

I've been testing this for a while and it seems to work. Thank you.

>  	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp, false);
>  	rcu_segcblist_advance(&sdp->srcu_cblist,
>  			      rcu_seq_current(&sp->srcu_gp_seq));


Sebastian