linux-kernel - Re: [PATCH 09/24] rcu/tree: cache specified number of objects

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Date:   Tue, 5 May 2020 13:03:20 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 09/24] rcu/tree: cache specified number of objects

On Mon, May 04, 2020 at 01:16:41PM -0700, Paul E. McKenney wrote:
> On Mon, May 04, 2020 at 09:51:28PM +0200, Uladzislau Rezki wrote:
> > > > > Since we don't care about traversing backwards, isn't it better to use llist
> > > > > for this usecase?
> > > > >
> > > > > I think Vlad is using locking as we're also tracking the size of the llist to
> > > > > know when to free pages. This tracking could suffer from the lost-update
> > > > > problem without any locking, 2 lockless llist_add happened simulatenously.
> > > > >
> > > > > Also if list_head is used, it will take more space and still use locking.
> > > >
> > > > Indeed, it would be best to use a non-concurrent singly linked list.
> > > 
> > > Ok cool :-)
> > > 
> > > Is it safe to say something like the following is ruled out? ;-) :-D
> > > #define kfree_rcu_list_add llist_add
> > > 
> > In that case i think it is better just to add a comment about using
> > llist_head. To state that it used as a singular list to save space
> > and the access is synchronized by the lock :)
> > 
> > IMHO.
> 
> But adding such a comment would be fine as well.
> 
Thank you Paul and Joel!

--
Vlad Rezki