Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to allow consolidation

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to allow consolidation
Date	2015-10-08 19:50 +0200
Message-ID	<qhnui-24d-13@gated-at.bofh.it> (permalink)
References	(5 earlier) <qgYcy-7Ng-7@gated-at.bofh.it> <qh04G-25n-5@gated-at.bofh.it> <qhfZL-88e-1@gated-at.bofh.it> <qhlsu-7EU-13@gated-at.bofh.it> <qhn1f-1wq-3@gated-at.bofh.it>
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On Thu, Oct 08, 2015 at 07:12:03PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 08, 2015 at 08:33:51AM -0700, Paul E. McKenney wrote:
> 
> > > > o	CPU B therefore moves up the tree, acquiring the parent
> > > > 	rcu_node structures' ->lock.  In so doing, it forces full
> > > > 	ordering against all prior RCU read-side critical sections
> > > > 	of all CPUs corresponding to all leaf rcu_node structures
> > > > 	subordinate to the current (non-leaf) rcu_node structure.
> > > 
> > > And here we iterate the tree and get another lock var involved, here the
> > > barrier upgrade will actually do something.
> > 
> > Yep.  And I am way too lazy to sort out exactly which acquisitions really
> > truly need smp_mb__after_unlock_lock() and which don't.  Besides, if I
> > tried to sort it out, I would occasionally get it wrong, and this would be
> > a real pain to debug.  Therefore, I simply do smp_mb__after_unlock_lock()
> > on all acquisitions of the rcu_node structures' ->lock fields.  I can
> > actually validate that!  ;-)
> 
> This is a whole different line of reasoning once again.
> 
> The point remains, that the sole purpose of the barrier upgrade is for
> the tree iteration, having some extra (pointless but harmless) instances
> does not detract from that.
> 
> > Fair enough, but I will be sticking to the simple coding rule that keeps
> > RCU out of trouble!
> 
> Note that there are rnp->lock acquires without the extra barrier though,
> so you seem somewhat inconsistent with your own rule.
> 
> See for example:
> 
> 	rcu_dump_cpu_stacks()
> 	print_other_cpu_stall()
> 	print_cpu_stall()
> 
> (did not do an exhaustive scan, there might be more)
> 
> and yes, that is 'obvious' debug code and not critical to the correct
> behaviour of the code, but it is a deviation from 'the rule'.

Which I need to fix, thank you.

							Thanx, Paul

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Thread

[PATCH tip/core/rcu 0/18] Expedited grace-period improvements for 4.4 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:30 +0200
  [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for synchronize_sched_expedited() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
    Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() Peter Zijlstra <peterz@infradead.org> - 2015-10-07 16:30 +0200
      Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 18:30 +0200
        Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() Peter Zijlstra <peterz@infradead.org> - 2015-10-08 11:10 +0200
          Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-08 17:10 +0200
            Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() Peter Zijlstra <peterz@infradead.org> - 2015-10-08 17:20 +0200
              Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-08 17:20 +0200
                Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() Josh Triplett <josh@joshtriplett.org> - 2015-10-08 20:10 +0200
                Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-09 02:20 +0200
                Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() Josh Triplett <josh@joshtriplett.org> - 2015-10-09 02:50 +0200
                Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for  synchronize_sched_expedited() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-09 06:00 +0200
  [PATCH tip/core/rcu 10/18] rcu: Stop silencing lockdep false positive for expedited grace periods "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 08/18] rcu: Make ->cpu_no_qs be a union for aggregate OR "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
    Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-06 22:30 +0200
      Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 23:00 +0200
        Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-07 10:00 +0200
          Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2015-10-07 10:50 +0200
            Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-07 13:10 +0200
              Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-07 14:00 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-07 14:10 +0200
                Re: Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation kbuild test robot <lkp@intel.com> - 2015-10-07 14:10 +0200
                Re: Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation kbuild test robot <lkp@intel.com> - 2015-10-07 14:10 +0200
                Re: Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-07 14:20 +0200
                Re: [kbuild-all] [PATCH tip/core/rcu 02/18] rcu: Move  rcu_report_exp_rnp() to allow consolidation Fengguang Wu <lkp@intel.com> - 2015-10-07 15:50 +0200
                Re: [kbuild-all] [PATCH tip/core/rcu 02/18] rcu: Move  rcu_report_exp_rnp() to allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-07 16:00 +0200
                Re: [kbuild-all] [PATCH tip/core/rcu 02/18] rcu: Move  rcu_report_exp_rnp() to allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-07 16:30 +0200
                Re: [kbuild-all] [PATCH tip/core/rcu 02/18] rcu: Move  rcu_report_exp_rnp() to allow consolidation Fengguang Wu <lkp@intel.com> - 2015-10-07 16:30 +0200
                Re: Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation kbuild test robot <lkp@intel.com> - 2015-10-07 14:20 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 17:20 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-08 12:30 +0200
              Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 17:20 +0200
          Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 16:40 +0200
            Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-07 16:50 +0200
              Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 18:50 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-08 11:50 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-08 17:40 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-08 19:20 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-08 19:50 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-09 02:20 +0200
                Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to  allow consolidation Peter Zijlstra <peterz@infradead.org> - 2015-10-09 10:50 +0200
  [PATCH tip/core/rcu 05/18] rcu: Move synchronize_sched_expedited() to combining tree "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 16/18] rcu: Add tasks to expedited stall-warning messages "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 12/18] cpu: Remove try_get_online_cpus() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 03/18] rcu: Consolidate tree setup for synchronize_rcu_expedited() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 13/18] rcu: Prepare for consolidating expedited CPU selection "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 07/18] rcu: Invert passed_quiesce and rename to cpu_no_qs "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm for RCU expedited grace period "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
    Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period Peter Zijlstra <peterz@infradead.org> - 2015-10-07 15:30 +0200
      Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 20:20 +0200
    Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period Peter Zijlstra <peterz@infradead.org> - 2015-10-07 15:40 +0200
      Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 17:50 +0200
    Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period Peter Zijlstra <peterz@infradead.org> - 2015-10-07 15:50 +0200
      Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 18:20 +0200
        Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period Peter Zijlstra <peterz@infradead.org> - 2015-10-08 11:10 +0200
    Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period Peter Zijlstra <peterz@infradead.org> - 2015-10-07 15:50 +0200
      Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm  for RCU expedited grace period "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 18:20 +0200
  [PATCH tip/core/rcu 09/18] rcu: Switch synchronize_sched_expedited() to IPI "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
    Re: [PATCH tip/core/rcu 09/18] rcu: Switch  synchronize_sched_expedited() to IPI Peter Zijlstra <peterz@infradead.org> - 2015-10-07 16:20 +0200
      Re: [PATCH tip/core/rcu 09/18] rcu: Switch  synchronize_sched_expedited() to IPI "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 18:30 +0200
  [PATCH tip/core/rcu 15/18] rcu: Add online/offline info to expedited stall warning message "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 14/18] rcu: Consolidate expedited CPU selection "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
  [PATCH tip/core/rcu 01/18] rcu: Use rsp->expedited_wq instead of sync_rcu_preempt_exp_wq "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
    [PATCH tip/core/rcu 06/18] rcu: Rename qs_pending to core_needs_qs "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200
    [PATCH tip/core/rcu 17/18] rcu: Enable stall warnings for synchronize_rcu_expedited() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:40 +0200

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