Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read_lock()

From	Josh Triplett <josh@joshtriplett.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read_lock()
Date	2015-10-06 19:50 +0200
Message-ID	<qgExc-4Ib-31@gated-at.bofh.it> (permalink)
References	<qgD85-2O1-3@gated-at.bofh.it> <qgD85-2O1-5@gated-at.bofh.it> <qgDhM-2Zi-17@gated-at.bofh.it> <qgE4a-49R-29@gated-at.bofh.it> <qgEnx-4wt-27@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Tue, Oct 06, 2015 at 10:36:46AM -0700, Paul E. McKenney wrote:
> On Tue, Oct 06, 2015 at 10:18:39AM -0700, Josh Triplett wrote:
> > On Tue, Oct 06, 2015 at 09:13:42AM -0700, Paul E. McKenney wrote:
> > > Currently, __srcu_read_lock() cannot be invoked from restricted
> > > environments because it contains calls to preempt_disable() and
> > > preempt_enable(), both of which can invoke lockdep, which is a bad
> > > idea in some restricted execution modes.  This commit therefore moves
> > > the preempt_disable() and preempt_enable() from __srcu_read_lock()
> > > to srcu_read_lock().  It also inserts the preempt_disable() and
> > > preempt_enable() around the call to __srcu_read_lock() in do_exit().
> > 
> > What restricted environments do you intend to invoke
> > __srcu_read_lock from?
> > 
> > This change seems fine, but I don't see any change in this patch series
> > that needs this, hence my curiosity.
> 
> Someone asked me for it, and now I cannot find it.  :-(
> 
> Something to the effect of when running unmapped during exception entry
> or something like that.  I guess one way to find out would be to remove
> the commit and see who complained, but on the other hand, it arguably
> makes more sense to have only the bare mechanism is __srcu_read_lock()
> and put the environmental protection into srcu_read_lock().

I agree; I just find the idea that someone would need to call
__srcu_read_lock rather than srcu_read_lock odd and worthy of further
understanding. :)
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Thread

[PATCH tip/core/rcu 0/13] Miscellaneous fixes for 4.4 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
  [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
    Re: [Kernel networking modules.]  OSI levels 2 & 3, Assistance - If anyone knows anyone in the US. North West region "John D Allen, Leveridge Systems INC"   <john.allen@leveridgesystems.com> - 2015-10-06 18:30 +0200
    Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers Josh Triplett <josh@joshtriplett.org> - 2015-10-06 18:50 +0200
      Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers Josh Triplett <josh@joshtriplett.org> - 2015-10-06 19:20 +0200
        Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers Josh Triplett <josh@joshtriplett.org> - 2015-10-06 19:50 +0200
        Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 19:50 +0200
          Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers Peter Zijlstra <peterz@infradead.org> - 2015-10-06 22:10 +0200
            Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 22:20 +0200
              Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers Peter Zijlstra <peterz@infradead.org> - 2015-10-06 23:00 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 23:10 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers Peter Zijlstra <peterz@infradead.org> - 2015-10-07 09:30 +0200
      Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for  Tiny and Tree RCU readers "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 19:20 +0200
  [PATCH tip/core/rcu 03/13] rcu: Use call_rcu_func_t to replace explicit type equivalents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
  [PATCH tip/core/rcu 01/13] sched: Export sched_setscheduler_nocheck "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
    [PATCH tip/core/rcu 09/13] rcu: Correct comment for values of ->gp_state field "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
    [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use lockless_dereference() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
    [PATCH tip/core/rcu 02/13] rcu: Use rcu_callback_t in call_rcu*() and friends "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
    [PATCH tip/core/rcu 12/13] rcu: Remove deprecated rcu_lockdep_assert() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
    [PATCH tip/core/rcu 06/13] rcu: Add online/offline info to stall warning message "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:20 +0200
      Re: [PATCH tip/core/rcu 06/13] rcu: Add online/offline info to stall  warning message Josh Triplett <josh@joshtriplett.org> - 2015-10-06 19:20 +0200
    [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:30 +0200
      Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() Josh Triplett <josh@joshtriplett.org> - 2015-10-06 19:20 +0200
        Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 19:40 +0200
          Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() Josh Triplett <josh@joshtriplett.org> - 2015-10-06 19:50 +0200
            Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 20:10 +0200
      Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() Peter Zijlstra <peterz@infradead.org> - 2015-10-06 22:10 +0200
        Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 22:20 +0200
          Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() Peter Zijlstra <peterz@infradead.org> - 2015-10-06 22:40 +0200
            Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 23:10 +0200
              Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() Peter Zijlstra <peterz@infradead.org> - 2015-10-07 09:30 +0200
                Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of  __srcu_read_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 16:20 +0200
    [PATCH tip/core/rcu 05/13] rcu: Eliminate panic when silly boot-time fanout specified "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:30 +0200
    [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 18:30 +0200
      Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() Josh Triplett <josh@joshtriplett.org> - 2015-10-06 19:30 +0200
        Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 19:40 +0200
          Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() Josh Triplett <josh@joshtriplett.org> - 2015-10-06 19:40 +0200
      Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() Peter Zijlstra <peterz@infradead.org> - 2015-10-06 22:30 +0200
        Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 23:10 +0200
          Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() Peter Zijlstra <peterz@infradead.org> - 2015-10-07 09:30 +0200
            Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-07 16:30 +0200
  Re: [PATCH tip/core/rcu 0/13] Miscellaneous fixes for 4.4 Josh Triplett <josh@joshtriplett.org> - 2015-10-06 19:30 +0200
    Re: [PATCH tip/core/rcu 0/13] Miscellaneous fixes for 4.4 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-06 19:40 +0200

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