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SubjectRe: Consolidating RCU-bh, RCU-preempt, and RCU-sched
On Fri, Jul 13, 2018 at 8:02 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> Hello!
>
> I now have a semi-reasonable prototype of changes consolidating the
> RCU-bh, RCU-preempt, and RCU-sched update-side APIs in my -rcu tree.
> There are likely still bugs to be fixed and probably other issues as well,
> but a prototype does exist.
>
> Assuming continued good rcutorture results and no objections, I am
> thinking in terms of this timeline:
>
> o Preparatory work and cleanups are slated for the v4.19 merge window.
>
> o The actual consolidation and post-consolidation cleanup is slated
> for the merge window after v4.19 (v5.0?). These cleanups include
> the replacements called out below within the RCU implementation
> itself (but excluding kernel/rcu/sync.c, see question below).
>
> o Replacement of now-obsolete update APIs is slated for the second
> merge window after v4.19 (v5.1?). The replacements are currently
> expected to be as follows:
>
> synchronize_rcu_bh() -> synchronize_rcu()
> synchronize_rcu_bh_expedited() -> synchronize_rcu_expedited()
> call_rcu_bh() -> call_rcu()
> rcu_barrier_bh() -> rcu_barrier()
> synchronize_sched() -> synchronize_rcu()
> synchronize_sched_expedited() -> synchronize_rcu_expedited()
> call_rcu_sched() -> call_rcu()
> rcu_barrier_sched() -> rcu_barrier()
> get_state_synchronize_sched() -> get_state_synchronize_rcu()
> cond_synchronize_sched() -> cond_synchronize_rcu()
> synchronize_rcu_mult() -> synchronize_rcu()
>
> I have done light testing of these replacements with good results.
>
> Any objections to this timeline?
>
> I also have some questions on the ultimate end point. I have default
> choices, which I will likely take if there is no discussion.
>
> o
> Currently, I am thinking in terms of keeping the per-flavor
> read-side functions. For example, rcu_read_lock_bh() would
> continue to disable softirq, and would also continue to tell
> lockdep about the RCU-bh read-side critical section. However,
> synchronize_rcu() will wait for all flavors of read-side critical
> sections, including those introduced by (say) preempt_disable(),
> so there will no longer be any possibility of mismatching (say)
> RCU-bh readers with RCU-sched updaters.
>
> I could imagine other ways of handling this, including:
>
> a. Eliminate rcu_read_lock_bh() in favor of
> local_bh_disable() and so on. Rely on lockdep
> instrumentation of these other functions to identify RCU
> readers, introducing such instrumentation as needed. I am
> not a fan of this approach because of the large number of
> places in the Linux kernel where interrupts, preemption,
> and softirqs are enabled or disabled "behind the scenes".
>
> b. Eliminate rcu_read_lock_bh() in favor of rcu_read_lock(),
> and required callers to also disable softirqs, preemption,
> or whatever as needed. I am not a fan of this approach
> because it seems a lot less convenient to users of RCU-bh
> and RCU-sched.
>
> At the moment, I therefore favor keeping the RCU-bh and RCU-sched
> read-side APIs. But are there better approaches?

Hello, Paul

Since local_bh_disable() will be guaranteed to be protected by RCU
and more general. I'm afraid it will be preferred over
rcu_read_lock_bh() which will be gradually being phased out.

In other words, keeping the RCU-bh read-side APIs will be a slower
version of the option A. So will the same approach for the RCU-sched.
But it'll still be better than the hurrying option A, IMHO.

Thanks,
Lai

>
> o How should kernel/rcu/sync.c be handled? Here are some
> possibilities:
>
> a. Leave the full gp_ops[] array and simply translate
> the obsolete update-side functions to their RCU
> equivalents.
>
> b. Leave the current gp_ops[] array, but only have
> the RCU_SYNC entry. The __INIT_HELD field would
> be set to a function that was OK with being in an
> RCU read-side critical section, an interrupt-disabled
> section, etc.
>
> This allows for possible addition of SRCU functionality.
> It is also a trivial change. Note that the sole user
> of sync.c uses RCU_SCHED_SYNC, and this would need to
> be changed to RCU_SYNC.
>
> But is it likely that we will ever add SRCU?
>
> c. Eliminate that gp_ops[] array, hard-coding the function
> pointers into their call sites.
>
> I don't really have a preference. Left to myself, I will be lazy
> and take option #a. Are there better approaches?
>
> o Currently, if a lock related to the scheduler's rq or pi locks is
> held across rcu_read_unlock(), that lock must be held across the
> entire read-side critical section in order to avoid deadlock.
> Now that the end of the RCU read-side critical section is
> deferred until sometime after interrupts are re-enabled, this
> requirement could be lifted. However, because the end of the RCU
> read-side critical section is detected sometime after interrupts
> are re-enabled, this means that a low-priority RCU reader might
> remain priority-boosted longer than need be, which could be a
> problem when running real-time workloads.
>
> My current thought is therefore to leave this constraint in
> place. Thoughts?
>
> Anything else that I should be worried about? ;-)
>
> Thanx, Paul
>
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