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Re: [PATCH tip/core/rcu 09/12] rcu: Make call_rcu_tasks() tolerate first call with irqs disabled

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On Thu, Jun 16, 2016 at 12:16:04AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 15, 2016 at 02:46:10PM -0700, Paul E. McKenney wrote:
> > Currently, if the very first call to call_rcu_tasks() has irqs disabled,
> > it will create the rcu_tasks_kthread with irqs disabled, which will
> > result in a splat in the memory allocator, which kthread_run() invokes
> > with the expectation that irqs are enabled.
> > 
> > This commit fixes this problem by deferring kthread creation if called
> > with irqs disabled.  The first call to call_rcu_tasks() that has irqs
> > enabled will create the kthread.
> > 
> > This bug was detected by rcutorture changes that were motivated by
> > Iftekhar Ahmed's mutation-testing efforts.
> 
> Seems fragile. What if someone manages to only use call_rcu_tasks() with
> IRQs disabled?

It would have to have users before that could possibly happen.  :-/
And it would not be hard to remove the fragility if needed by setting
up a workqueue, possibly mediated by a timer or whatever.  But it is
hard to motivate myself to do so in advance of users.  For that matter...

Steven, is call_rcu_tasks() needed, or should I just rip it out?

							Thanx, Paul

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[PATCH tip/core/rcu 0/12] Miscellaneous RCU fixes for 4.8 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
  [PATCH tip/core/rcu 10/12] rcu: Fix a typo in a comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
  [PATCH tip/core/rcu 07/12] rcu: No ordering for rcu_assign_pointer() of NULL "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
    Re: [PATCH tip/core/rcu 07/12] rcu: No ordering for  rcu_assign_pointer() of NULL Peter Zijlstra <peterz@infradead.org> - 2016-06-16 00:10 +0200
      Re: [PATCH tip/core/rcu 07/12] rcu: No ordering for  rcu_assign_pointer() of NULL Peter Zijlstra <peterz@infradead.org> - 2016-06-16 00:20 +0200
        Re: [PATCH tip/core/rcu 07/12] rcu: No ordering for  rcu_assign_pointer() of NULL "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-16 00:50 +0200
  [PATCH tip/core/rcu 02/12] rcu: Fix outdated hotplug-exclusion comment in rcu_gp_init() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
  [PATCH tip/core/rcu 03/12] rcu: Remove some superfluous lines "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
  [PATCH tip/core/rcu 04/12] rcu: Move expedited code from tree.c to tree_exp.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
    Re: [PATCH tip/core/rcu 04/12] rcu: Move expedited code from tree.c  to tree_exp.h Peter Zijlstra <peterz@infradead.org> - 2016-06-16 00:10 +0200
      Re: [PATCH tip/core/rcu 04/12] rcu: Move expedited code from tree.c  to tree_exp.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-16 00:20 +0200
    Re: [PATCH tip/core/rcu 04/12] rcu: Move expedited code from tree.c  to tree_exp.h Pranith Kumar <bobby.prani@gmail.com> - 2016-06-17 17:50 +0200
      Re: [PATCH tip/core/rcu 04/12] rcu: Move expedited code from tree.c  to tree_exp.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-17 19:50 +0200
  [PATCH tip/core/rcu 12/12] rcu: Correctly handle sparse possible cpus "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
  [PATCH tip/core/rcu 08/12] rcu: Disable TASKS_RCU for usermode Linux "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
    Re: [PATCH tip/core/rcu 08/12] rcu: Disable TASKS_RCU for usermode  Linux Richard Weinberger <richard@nod.at> - 2016-06-15 23:50 +0200
  [PATCH tip/core/rcu 06/12] rcu: Document RCU_NONIDLE() restrictions in comment header "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
  [PATCH tip/core/rcu 11/12] rcu: sysctl: Panic on RCU Stall "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-15 23:50 +0200
  [PATCH tip/core/rcu 05/12] rcu: Move expedited code from tree_plugin.h to tree_exp.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-16 00:00 +0200
  [PATCH tip/core/rcu 09/12] rcu: Make call_rcu_tasks() tolerate first call with irqs disabled "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-16 00:00 +0200
    Re: [PATCH tip/core/rcu 09/12] rcu: Make call_rcu_tasks() tolerate  first call with irqs disabled Peter Zijlstra <peterz@infradead.org> - 2016-06-16 00:20 +0200
      Re: [PATCH tip/core/rcu 09/12] rcu: Make call_rcu_tasks() tolerate  first call with irqs disabled "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-16 01:00 +0200
    Re: [PATCH tip/core/rcu 09/12] rcu: Make call_rcu_tasks() tolerate  first call with irqs disabled Peter Zijlstra <peterz@infradead.org> - 2016-06-16 00:20 +0200
      Re: [PATCH tip/core/rcu 09/12] rcu: Make call_rcu_tasks() tolerate  first call with irqs disabled "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-16 01:00 +0200

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