Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers

From	Josh Triplett <josh@joshtriplett.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers
Date	2015-10-06 19:50 +0200
Message-ID	<qgExc-4Ib-25@gated-at.bofh.it> (permalink)
References	(2 earlier) <qgD86-2O1-23@gated-at.bofh.it> <qgDB8-3mf-13@gated-at.bofh.it> <qgE49-49R-21@gated-at.bofh.it> <qgE49-49R-19@gated-at.bofh.it> <qgExc-4Ib-27@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Tue, Oct 06, 2015 at 10:42:04AM -0700, Paul E. McKenney wrote:
> On Tue, Oct 06, 2015 at 10:16:30AM -0700, Josh Triplett wrote:
> > On Tue, Oct 06, 2015 at 10:01:01AM -0700, Paul E. McKenney wrote:
> > > On Tue, Oct 06, 2015 at 09:44:45AM -0700, Josh Triplett wrote:
> > > > On Tue, Oct 06, 2015 at 09:13:39AM -0700, Paul E. McKenney wrote:
> > > > > From: Boqun Feng <boqun.feng@gmail.com>
> > > > > 
> > > > > Because preempt_disable() maps to barrier() for non-debug builds,
> > > > > it forces the compiler to spill and reload registers.  Because Tree
> > > > > RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
> > > > > barrier() instances generate needless extra code for each instance of
> > > > > rcu_read_lock() and rcu_read_unlock().  This extra code slows down Tree
> > > > > RCU and bloats Tiny RCU.
> > > > > 
> > > > > This commit therefore removes the preempt_disable() and preempt_enable()
> > > > > from the non-preemptible implementations of __rcu_read_lock() and
> > > > > __rcu_read_unlock(), respectively.  However, for debug purposes,
> > > > > preempt_disable() and preempt_enable() are still invoked if
> > > > > CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside
> > > > > atomic sections in non-preemptible kernels.
> > > > > 
> > > > > This is based on an earlier patch by Paul E. McKenney, fixing
> > > > > a bug encountered in kernels built with CONFIG_PREEMPT=n and
> > > > > CONFIG_PREEMPT_COUNT=y.
> > > > 
> > > > This also adds explicit barrier() calls to several internal RCU
> > > > functions, but the commit message doesn't explain those at all.
> > > 
> > > To compensate for them being removed from rcu_read_lock() and
> > > rcu_read_unlock(), but yes, I will update.
> > 
> > That much seemed clear from the comments, but that doesn't explain *why*
> > those functions need barriers of their own even though rcu_read_lock()
> > and rcu_read_unlock() don't.
> 
> Ah.  The reason is that Tiny RCU and Tree RCU (the !PREEMPT ones) act
> by implicitly extending (and, if need be, merging) the RCU read-side
> critical sections to include all the code between successive quiescent
> states, for example, all the code between a pair of calls to schedule().
> 
> Therefore, there need to be barrier() calls in the quiescent-state
> functions.  Some could be argued to be implicitly present due to
> translation-unit boundaries, but paranoia and all that.
> 
> Would adding that sort of explanation help?

Yes, it would.
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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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