linux-kernel - Re: [PATCH v2 4/4] task: RCUify the assignment of rq->curr

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Message-ID: <87v9tta03l.fsf@x220.int.ebiederm.org>
Date:   Sun, 15 Sep 2019 13:25:02 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 4/4] task: RCUify the assignment of rq->curr

ebiederm@...ssion.com (Eric W. Biederman) writes:

> "Paul E. McKenney" <paulmck@...nel.org> writes:
>
>> So this looks good in and of itself, but I still do not see what prevents
>> the unfortunate sequence of events called out in my previous email.
>> On the other hand, if ->rcu and ->rcu_users were not allocated on top
>> of each other by a union, I would be happy to provide a Reviewed-by.
>>
>> And I am fundamentally distrusting of a refcount_dec_and_test() that
>> is immediately followed by code that clobbers the now-zero value.
>> Yes, this does have valid use cases, but it has a lot more invalid
>> use cases.  The valid use cases have excluded all increments somehow
>> else, so that the refcount_dec_and_test() call's only job is to
>> synchronize between concurrent calls to put_task_struct_rcu_user().
>> But I am not seeing the "excluded all increments somehow".
>>
>> So, what am I missing here?
>
> Probably only that the users of the task_struct in this sense are now
> quite mature.
>
> The two data structures that allow rcu access to the task_struct are
> the pid hash and the runqueue.    The practical problem is that they
> have two very different lifetimes.  So we need some kind of logic that
> let's us know when they are both done.  A recount does that job very
> well.
>
> Placing the recount on the same storage as the unused (at that point)
> rcu_head removes the need to be clever in other ways to avoid bloating
> the task_struct.
>
> If you really want a reference to the task_struct from rcu context you
> can just use get_task_struct.  Because until the grace period completes
> it is guaranteed that the task_struct has a positive count.
>
> Right now I can't imagine a use case for wanting to increase rcu_users
> anywhere or to decrease rcu_users except where we do.  If there is such
> a case most likely it will increase the reference count at
> initialization time.
>
> If anyone validly wants to increment rcu_users from an rcu critical
> section we can move it out of the union at that time.

Paul were you worrying about incrementing rcu_users because Frederic
Weisbecker brought the concept up earlier in the review?

It was his confusion that the point of rcu_users was so that it could
be incremented from an rcu critical section.  That definitely is not
the point of rcu_users.

If you were wondering about someone messing with rcu_users from an rcu
critical region independently that does suggest the code could use
a "comment saying don't do that!"  Multiple people getting confused
about the purpose of a reference count independently does suggest there
is a human factor problem in there somewhere.

Eric