linux-kernel - Re: [PATCH] kernel: events: core: Replace rcu_assign_pointer() with RCU_INIT

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Message-ID: <20140926145335.GI4140@worktop.programming.kicks-ass.net>
Date:	Fri, 26 Sep 2014 16:53:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Andreea-Cristina Bernat <bernat.ada@...il.com>, paulus@...ba.org,
	mingo@...hat.com, acme@...nel.org, linux-kernel@...r.kernel.org,
	stephen@...workplumber.org, eric.dumazet@...il.com
Subject: Re: [PATCH] kernel: events: core: Replace rcu_assign_pointer() with
 RCU_INIT_POINTER()

On Fri, Sep 12, 2014 at 09:30:55AM -0700, Paul E. McKenney wrote:
> On Wed, Sep 10, 2014 at 03:12:51PM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 09, 2014 at 09:16:48AM -0700, Paul E. McKenney wrote:
> > > On Tue, Sep 09, 2014 at 11:42:35AM +0200, Peter Zijlstra wrote:
> > 
> > > > Paul, why not do something like the below and do away with all this
> > > > nonsense?
> > > 
> > > We used to do that, but the compilers became uncooperative, despite
> > > Stephen Hemminger's best efforts.
> > 
> > Happen to have a link handy to that thread so I can educate myself?
> 
> After a bit of software archeology, here you go...
> 
> Commit d322f45ceed52 (rcu: Make rcu_assign_pointer() unconditionally
> insert a memory barrier) made this change.  According to the commit
> log:
> 
>     Recent changes to gcc give warning messages on rcu_assign_pointers()'s
>     checks that allow it to determine when it is OK to omit the memory
>     barrier.  Stephen Hemminger tried a number of gcc tricks to silence
>     this warning, but #pragmas and CPP macros do not work together in the
>     way that would be required to make this work.
> 
> This was applied in 2011, and searching LKML during that time gives
> the following:  https://lkml.org/lkml/2011/7/29/305
> 
> And in this LKML post, Stephen Hemminger wrote:
> 
> 	Gcc now generates warnings from rcu_assign_pointer when passed the
> 	address of something for example:
> 		rcu_assign_pointer(dev_queue->qdisc, &noop_qdisc);
> 	This warning is harmless and should be surpressed but there maybe
> 	other cases where we want that Gcc warning.
> 
> 	I tried various combinations of in rcu_assign_pointer macro
> 	  #pragma GCC diagnostic push
> 	  #pragma GCC diagnostic ignored "-Wlogical-op"
> 	  ...
> 	  #pragma GCC diagnostic pop
> 	but macro's and pragma's don't nest with the correct scope for
> 	this.
> 
> 	Maybe some one with more Gcc foo and time to waste could take
> 	a crack at it.
> 
> Adding Stephen on CC in case he remembers more.

So I read all that and am still left wondering WTF the problem was :/

I googled a bit and found this thread:

  https://lkml.org/lkml/2014/3/24/39

Where Michael actually has two 'fixes' to the problem. Instead we
continue clutter the API with this silly RCU_INIT_POINTER stuff, totally
sad.

But nowhere have I found the actual warning GCC gives, I suppose I can
go change my local tree and compile some code to obtain it, but that
seems backwards. The patch 'working' around that should have mentioned
it and explained why the warning is bogus or not.
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