linux-kernel - Re: [PATCH RFC tip/core/rcu] SRCU rewrite

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Message-Id: <20161117134957.GW3612@linux.vnet.ibm.com>
Date:   Thu, 17 Nov 2016 05:49:57 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, dipankar@...ibm.com,
        akpm@...ux-foundation.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>, edumazet@...gle.com,
        dvhart@...ux.intel.com,
        Frédéric Weisbecker <fweisbec@...il.com>,
        oleg@...hat.com, bobby.prani@...il.com, ldr709@...il.com
Subject: Re: [PATCH RFC tip/core/rcu] SRCU rewrite

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@...ux.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.

							Thanx, Paul