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

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Date:   Thu, 17 Nov 2016 09:42:09 -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>,
        Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
        Frédéric Weisbecker <fweisbec@...il.com>,
        oleg@...hat.com, pranith kumar <bobby.prani@...il.com>,
        ldr709@...il.com
Subject: Re: [PATCH RFC tip/core/rcu] SRCU rewrite

On Thu, Nov 17, 2016 at 11:55:07PM +0800, Lai Jiangshan wrote:
> On Thu, Nov 17, 2016 at 10:45 PM, Boqun Feng <boqun.feng@...il.com> wrote:
> > On Thu, Nov 17, 2016 at 06:38:29AM -0800, Paul E. McKenney wrote:
> >> 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@...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.
> >>
> >> And one area needing special thought is the call to __srcu_read_lock()
> >> and __srcu_read_unlock() in do_exit().
> >>
> >
> > So before commit 49f5903b473c5, we don't have the read of ->completed in
> > preemption disable section?
> >
> > And following "git blame", I found commit 7a6b55e7108b3 ;-)
> 
> Ouch, it shows 7a6b55e7108b3 at least has a bug in the comments about NR_CPUS.
> 
> we should focus on the total number of all active readers instead the number
> of the readers using the old index that haven't incremented the counters.
> the later is smaller than the prior one which is smaller than the ULONG_MAX/4
> or even smaller. so that we can simplify the comments.
> 
> +        * Note that the sum of the ->lock_count[]s cannot increment enough
> +        * times to overflow and end up equal the sum of the ->unlock_count[]s,
> +        * even too much readers using the old index that haven't incremented
> +        * ->lock_count[] yet, as long as there are at most ULONG_MAX/4
> +        * readers at a time.  Therefore, the only way that the return values of
> +        * the two calls to srcu_readers_(un)lock_idx() can be equal is if there
> +        * are no active readers using this index.

I would welcome a patch making the limitations simpler and more accurate.
That is, once we work out exactly what those limitations are.  ;-)

							Thanx, Paul