Re: [PATCH RFC tip/core/rcu 1/6] rcu: Stabilize use of num_online_cpus() for GP short circuit

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 04/24/2012 10:20 PM, Paul E. McKenney wrote:

> On Tue, Apr 24, 2012 at 09:05:20PM +0530, Srivatsa S. Bhat wrote:
> 
>>
>>> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
>>>
>>> The rcu_blocking_is_gp() function tests to see if there is only one
>>> online CPU, and if so, synchronize_sched() and friends become no-ops.
>>> However, for larger systems, num_online_cpus() scans a large vector,
>>
>>
>> Venki had posted a patch to optimize that by using a variable, so that we
>> don't calculate the value each and every time.
>> http://thread.gmane.org/gmane.linux.kernel/1240569/focus=1246659
>>
>> However, unfortunately there was some confusion around that patch and
>> even though it made it to akpm's tree and stayed there briefly, it didn't
>> go upstream. Venki had attempted to resolve the confusion here:
>> http://thread.gmane.org/gmane.linux.kernel/1240569/focus=1260702
> 
> Having a single variable tracking the online state would be good,
> but as you say it isn't there yet.
> 
>>> and might be preempted while doing so.  While preempted, any number
>>> of CPUs might come online and go offline, potentially resulting in
>>> num_online_cpus() returning 1 when there never had only been one
>>> CPU online.  This would result in a too-short RCU grace period, which
>>> could in turn result in total failure.
>>
>>
[...]

> 
> The problematic case is instead the one where we were SMP throughout,
> but rcu_blocking_is_gp() mistakenly decides that we were UP.  For example,
> consider the following sequence of events, based on the commit log's
> sentence "While preempted, any number of CPUs might come online and go
> offline, potentially resulting in num_online_cpus() returning 1 when
> there never had only been one CPU online":
> 


Oh, I didn't think in the direction illustrated below when reading that
sentence.. :-(

> o	CPUs 100 and 150 are initially online, with a long-running RCU
> 	read-side critical section on CPU 100 and rcu_blocking_is_gp()
> 	initially running on CPU 150.
> 
> o	The rcu_blocking_is_gp() function checks the bits for CPUs
> 	0-63, and counts zero online CPUs.
> 
> o	CPU 1 comes online.
> 
> o	The rcu_blocking_is_gp() function checks the bits for CPUs
> 	64-127, and counts one online CPUs, for a total thus far
> 	of one CPU online..
> 
> o	CPU 150 goes offline.  Ah, but it cannot do this, because
> 	this is non-preemptible RCU, which means that the RCU
> 	read-side critical section has disabled preemption on
> 	CPU 100, which prevents CPU 150 from going offline, which
> 	prevents this scenario from occurring.
> 
> 	So, yes, rcu_blocking_is_gp() can be fooled into incorrectly
> 	stating that the system has only one CPU (or even that it has
> 	only zero CPUs), but not while there is actually a non-preemptible
> 	RCU read-side critical section running.  Yow!
>


Awesome :-)

 
> 	I clearly had not thought this change through far enough,
> 	thank you for calling it to my attention!
> 
> So I could replace this patch with a patch that adds a comment
> explaining why this works.


Yes, that would be great..

>  Though this patch might be simpler and
> easier to understand.  ;-)


Oh well, but I completely missed the intention behind the patch!
So I guess a comment would be better ;-)

>  But not so good for real-time response
> on large systems, I suppose.
> 
> And rcu_blocking_is_gp() is called only from synchronize_sched() and
> synchronize_rcu_bh(), so it is safe for all current callers.  But it is
> defined publicly, so I should move it to somewhere like kernel/rcutree.c
> to keep new users from being fatally confused and disappointed.
> 
> I can also change the comparison from "num_online_cpus() == 1" to
> "num_online_cpus() <= 1".  That should at least serve as a caution to
> people who might attempt to use it where it shouldn't be used.  ;-)
> 


Hehe, yeah!

Regards,
Srivatsa S. Bhat