linux-kernel - Re: [PATCH 00/13] Rename k[v]free_rcu() single argument to k[v]free_rcu

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Date:   Wed, 15 Mar 2023 18:26:28 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>, Jens Axboe <axboe@...nel.dk>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        Philipp Reisner <philipp.reisner@...bit.com>,
        Bryan Tan <bryantan@...are.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Bob Pearson <rpearsonhpe@...il.com>,
        Ariel Levkovich <lariel@...dia.com>,
        "Theodore Ts'o" <tytso@....edu>, Julian Anastasov <ja@....bg>
Subject: Re: [PATCH 00/13] Rename k[v]free_rcu() single argument to
 k[v]free_rcu_mightsleep()

On Wed, 15 Mar 2023 18:08:19 -0400
Joel Fernandes <joel@...lfernandes.org> wrote:

> I am doubtful there may be a future where it does not sleep. Why?
> Because you need an rcu_head *somewhere*. Unlike with debubojects,
> which involves a lock-free per-CPU pool and a locked global pool, and
> has the liberty to shutdown if it runs out of objects -- in RCU code
> it doesn't have that liberty and it has to just keep working.  The
> kfree_rcu code does have pools of rcu_head as well, but that is not
> thought to be enough to prevent OOM when memory needs to be given
> back.  AFAIK -- the synchronize_rcu() in there is a last resort and
> undesirable (supposed to happen only when running out of
> objects/memory).

And everything you said above is still implementation, and the user of
kvfree_rcu() doesn't care.

The only thing different about the two cases is that one is headless.

> 
> Also "mightsleep" means just that -- *might*.  That covers the fact
> that sleeping may not happen ;-).

Yes, and even though you are doubtful of it not ever having a non-sleep
implementation, there is still a chance that there might be something
someday.

> 
> This is just my opinion and I will defer to Uladzislau, Paul and you
> on how to proceed. Another option is "cansleep" which has the same
> number of characters as headless. I don't believe expecting users to
> read comments is practical, since we did already have comments and
> there was a bug in the usage that triggered this whole series.

The point of "headless" is that is the rational for this version of
kvfree_rcu(). It doesn't have a head. That's an API name that users care
about.

Why not call it kvfree_rcu_alloc() ? It allocates right?

We have might_sleep() in lots of places. In fact, the default is things
might sleep. We don't need to call it out. That's what the might_sleep()
call is for. Usually it's the non sleep version that is special.

We could call the normal kvfree_rcu() "kvfree_rcu_inatomic()" ;-)

But I guess that would be a bigger change.

-- Steve