linux-kernel - Re: [RFC PATCH 1/1] rcu: use atomic_read(v) instead of atomic_add

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Date:	Wed, 16 Jul 2014 21:50:54 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Josh Triplett <josh@...htriplett.org>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/1] rcu: use atomic_read(v) instead of atomic_add_return(0,
 v)

On 07/16/2014 09:14 AM, Paul E. McKenney wrote:
> On Mon, Jul 14, 2014 at 09:27:00AM -0400, Pranith Kumar wrote:
>> On Sat, Jul 12, 2014 at 8:08 AM, Paul E. McKenney wrote:
>>>
>>> They ensure that any RCU read-side critical sections that took place before
>>> the current (or previous) idle/userspace period on the remote CPU in
>>> question are seen as having completed before the completion of the current
>>> grace period.  It also ensures that any RCU read-side critical sections
>>> that extend beyond the end of the current grace period (thus starting
>>> after the current (or previous) idle/userspace period) see any updates
>>> that were carried out before the beginning of the current grace period.
>>>
>>> Of course, that is also the purpose of many of RCU's memory barriers,
>>> so this probably doesn't help much.  An alternative explanation is that
>>> it ensures a coherent view of the ->dynticks counter, but I am quite
>>> sure that this helps even less.
>>>
>>> So here is the problem we are having:
>>>
>>> The dyntick_save_progress_counter() and rcu_implicit_dynticks_qs()
>>> functions are really bad places to start reading the RCU code.  I would
>>> say that starting there is like learning to swim by diving into the deep
>>> end of a swimming pool, but that doesn't really capture it.  Instead,
>>> it is more like learning to swim by diving from the top of this waterfall:
>>>
>>> http://blog.pacificnorthwestphotography.com/wp-content/uploads/2011/03/54.jpg
>>>
>>> To understand these functions, you will first need to understand how
>>> the rest of RCU works.  These functions are tiny cogs in RCU's energy
>>> efficiency optimization mechanism, which fits into the larger grace-period
>>> detection mechanism.  The purpose of the two atomic operations is to
>>> preserve the memory-ordering guarantees called out in the docbook header
>>> comments for call_rcu() and synchronize_rcu(), and I must confess that
>>> it is not clear to me that you actually read these header comments.
>>> Even so, these two functions interact with lots of other accesses to
>>> implement these guarantees -- so again, it is really really difficult
>>> to understand these two functions in isolation.
>>>
>>> Please see the end of this message for my suggested order of learning
>>> the RCU code.  A study plan, if you will.
>>
>> This guide helps a lot, thank you for the detailed study plan. I will
>> make sure to go slow and steady. :)
> 
> Best of everything with it!
> 

Thanks a lot, Paul, for taking the time to help me understand. Hopefully one of
these days, after reading the study list, I actually will :)

--
Pranith.
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