linux-kernel - Re: [PATCH 3/3] kvfree_rcu: use migrate

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Date:   Wed, 20 Jan 2021 20:45:54 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()

On 2021-01-20 17:21:48 [+0100], Uladzislau Rezki (Sony) wrote:
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3489,10 +3489,12 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
>  			(*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
>  		bnode = get_cached_bnode(*krcp);
>  		if (!bnode && can_alloc) {
> +			migrate_disable();
>  			krc_this_cpu_unlock(*krcp, *flags);

Why is krc_this_cpu_unlock() defined as
| static inline void
| krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
| {
|         raw_spin_unlock(&krcp->lock);
|         local_irq_restore(flags);
| }

instead of raw_spin_unlock_irqrestore()?
Should something with the locked section trigger a scheduling event by
setting TIF_NEED_RESCHED then there will be no scheduling event on
unlock. It will be delayed until a later "random" preempt_enable().

raw_spin_unlock_irqrestore() will reschedule if the flag is set,
local_irq_restore() will not.

Sebastian