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

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Message-ID: <72a817c7-52fe-4a2d-8191-6f134ecfecaf@paulmck-laptop>
Date:   Wed, 15 Mar 2023 19:52:35 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.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>,
        Julian Anastasov <ja@....bg>
Subject: Re: [PATCH 00/13] Rename k[v]free_rcu() single argument to
 k[v]free_rcu_mightsleep()

On Wed, Mar 15, 2023 at 09:25:16PM -0400, Theodore Ts'o wrote:
> On Wed, Mar 15, 2023 at 06:08:19PM -0400, Joel Fernandes wrote:
> > 
> > I am doubtful there may be a future where it does not sleep. Why?
> > Because you need an rcu_head *somewhere*.
> 
> I think the real problem was that this won't sleep:
> 
>    kfree_rcu(ptr, rhf);
> 
> While this *could* sleep:
> 
>    kfree_rcu(ptr);
> 
> So the the original sin was to try to make the same mistake that C++
> did --- which is to think that it's good to have functions that have
> the same name but different function signatures, and in some cases,
> different semantic meanings because they have different implementations.

Guilty to charges as read.  ;-)

> Personally, this is why I refuse to use C++ for any of my personal
> projects --- this kind of "magic" looks good, but it's a great way to
> potentially shoot yourself (or worse, your users) in the foot.
> 
> So separating out the two-argument kfree_rcu() from the one-argument
> kfree_rcu(), by renaming the latter to something else is IMHO, a
> Really F***** Good Idea.  So while, sure, kfree_rcu_mightsleep() might
> be a little awkward, the name documents the potential landmind
> involved with using that function, that's a good thing.  Because do
> you really think users will always conscientiously check the
> documentation and/or the implementation before using the interface?  :-)
> 
> If you hate that name, one other possibility is to try to use the
> two-argument form kfree_rcu() and arrange to *have* a rcu_head in the
> structure.  That's going to be better from a performance perspective,
> and thus kinder to the end user than using rcu_synchronize().

The original reason for single-argument kvfree_rcu() was to avoid
the need for that rcu_head.  The use case was a small data structure
with an extremely high population.

							Thanx, Paul