linux-kernel - Re: [PATCH 15/16] rcu/tree: Allocate a page when caller is preemptible

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Message-ID: <20201104143651.GA2313912@google.com>
Date:   Wed, 4 Nov 2020 09:36:51 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 15/16] rcu/tree: Allocate a page when caller is
 preemptible

On Wed, Nov 04, 2020 at 12:39:31PM +0100, Uladzislau Rezki wrote:
[..] 
> > > -	if (!krcp->bkvhead[idx] ||
> > > -			krcp->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> > > -		bnode = get_cached_bnode(krcp);
> > > -		/* Switch to emergency path. */
> > > +	if (!(*krcp)->bkvhead[idx] ||
> > > +			(*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> > > +		bnode = get_cached_bnode(*krcp);
> > > +		if (!bnode && can_alloc_page) {
> > 
> > I think you can directly put preemptible() here with a comment saying
> > allocate only if preemptible and get rid of can_alloc_page.
> > 
> Not really. We check preemtable() before acquiring the internal lock,
> otherwise it will always return "false". Thus, it is checked on the
> entry in the beginning.

You are right. Sorry. Sounds good to me.

 - Joel