Date: Thu, 17 Nov 2016 06:38:29 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, dipankar@in.ibm.com,
        akpm@linux-foundation.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>, edumazet@google.com,
        dvhart@linux.intel.com,
        =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
        oleg@redhat.com, bobby.prani@gmail.com, ldr709@gmail.com
Subject: Re: [PATCH RFC tip/core/rcu] SRCU rewrite
Reply-To: paulmck@linux.vnet.ibm.com
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On Thu, Nov 17, 2016 at 05:49:57AM -0800, Paul E. McKenney wrote:
> On Thu, Nov 17, 2016 at 08:18:51PM +0800, Lai Jiangshan wrote:
> > On Tue, Nov 15, 2016 at 10:37 PM, Paul E. McKenney
> > <paulmck@linux.vnet.ibm.com> wrote:
> > > On Tue, Nov 15, 2016 at 09:44:45AM +0800, Boqun Feng wrote:
> > 
> > >>
> > >> __srcu_read_lock() used to be called with preemption disabled. I guess
> > >> the reason was because we have two percpu variables to increase. So with
> > >> only one percpu right, could we remove the preempt_{dis,en}able() in
> > >> srcu_read_lock() and use this_cpu_inc() here?
> > >
> > > Quite possibly...
> > >
> > 
> > it will be nicer if it is removed.
> > 
> > The reason for the preemption-disabled was also because we
> > have to disallow any preemption between the fetching of the idx
> > and the increasement. so that we have at most NR_CPUS worth
> > of readers using the old index that haven't incremented the counters.
> > 
> > if we remove the preempt_{dis,en}able(). we must change the
> > "NR_CPUS" in the comment into ULONG_MAX/4. (I assume
> > one on-going reader needs at least need 4bytes at the stack). it is still safe.
> > 
> > but we still need to think more if we want to remove the preempt_{dis,en}able().
> 
> Good points!  Agreed, any change in the preemption needs careful thought
> and needs to be a separate patch.

And one area needing special thought is the call to __srcu_read_lock()
and __srcu_read_unlock() in do_exit().

							Thanx, Paul