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

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Date:   Thu, 23 Feb 2023 12:36:57 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     paulmck@...nel.org
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        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>,
        Steven Rostedt <rostedt@...dmis.org>,
        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 2/23/23 11:31 AM, Paul E. McKenney wrote:
> On Thu, Feb 23, 2023 at 07:57:13AM -0700, Jens Axboe wrote:
>> On 2/1/23 8:08 AM, Uladzislau Rezki (Sony) wrote:
>>> This small series is based on Paul's "dev" branch. Head is 6002817348a1c610dc1b1c01ff81654cdec12be4
>>> it renames a single argument of k[v]free_rcu() to its new k[v]free_rcu_mightsleep() name.
>>>
>>> 1.
>>> The problem is that, recently we have run into a precedent when
>>> a user intended to give a second argument to kfree_rcu() API but
>>> forgot to do it in a code so a call became as a single argument
>>> of kfree_rcu() API.
>>>
>>> 2.
>>> Such mistyping can lead to hidden bags where sleeping is forbidden.
>>>
>>> 3.
>>> _mightsleep() prefix gives much more information for which contexts
>>> it can be used for.
>>
>> This patchset seems weird to me. We have a LOT of calls that might
>> sleep, yet we don't suffix them all with _mightsleep(). Why is this
>> any different? Why isn't this just a might_sleep() call in the
>> actual helper, which will suffice for checkers and catch it at
>> runtime as well.
> 
> Fair enough, and the situation that this patchset is addressing is also a
> bit unusual.  This change was requested by Eric Dumazet due to a situation
> where someone forgot the optional second argument to kfree_rcu().  Now,
> you are right that this would be caught if invoked from a non-sleepable
> context, but there are also cases where sleeping is legal, but where the
> occasional wait for an RCU grace period would be a problem.  The checkers
> cannot easily catch this sort of thing, and hence the change in name.
> 
> Hey, the combined one/two-argument form seemed like a good idea at
> the time!  ;-)

Hah, not sure what you were smoking back then!

-- 
Jens Axboe