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.

        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?

o       How should kernel/rcu/sync.c be handled?  Here are some

        a.      Leave the full gp_ops[] array and simply translate
                the obsolete update-side functions to their RCU

        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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