On Fri, Jul 13, 2018 at 8:02 AM, Paul E. McKenney <[email protected]> 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 >
