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Date: Tue, 6 Oct 2015 10:46:31 -0700 From: Josh Triplett <josh@...htriplett.org> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: linux-kernel@...r.kernel.org, mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com, Boqun Feng <boqun.feng@...il.com> Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers 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@...il.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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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