linux-kernel - Re: [patch 01/15] Kernel Tracepoints

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Message-ID: <20080802001711.GC6733@linux.vnet.ibm.com>
Date:	Fri, 1 Aug 2008 17:17:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <mhiramat@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Hideo AOKI <haoki@...hat.com>,
	Takashi Nishiie <t-nishiie@...css.fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Subject: Re: [patch 01/15] Kernel Tracepoints

On Sat, Aug 02, 2008 at 02:03:57AM +0200, Peter Zijlstra wrote:
> On Fri, 2008-08-01 at 14:10 -0700, Paul E. McKenney wrote:
> 
> > Yeah, I was thinking in terms of rcu_dereference() working with both
> > rcu_assign_pointer() and an as-yet-mythical rcu_assign_index().  Perhaps
> > this would be a good time to get better names:
> > 
> > Current:	rcu_assign_pointer()	rcu_dereference()
> > New Pointers:	rcu_publish_pointer()	rcu_subscribe_pointer()
> > New Indexes:	rcu_publish_index()	rcu_subscribe_index()
> 
> Is it really worth the effort, splitting it out into these two cases?

Either we should split out into the pointer/index cases, or the
definition of rcu_assign_pointer() should probably lose its current
check for NULL...

> > And, while I am at it, work in a way of checking for either being in
> > the appropriate RCU read-side critical section and/or having the
> > needed lock/mutex/whatever held -- something I believe PeterZ was
> > prototyping some months back.
> 
> Yeah - I have (bitrotted a bit, but should be salvageable) lockdep
> annotations for rcu_dereference().
> 
> The problem with them is the huge amount of false positives.. Take for
> example the Radix tree code, its perfectly fine to use the radix tree
> code without RCU - say you do the old rwlock style, still it uses
> rcu_dereference().
> 
> I never figured out a suitable way to annotate that.

My thought was to add a second argument that contained a boolean.  If
the rcu_dereference() was either within an RCU read-side critical
section on the one hand or if the boolean evaluated to "true" on the
other, then no assertion.  This would require SPIN_LOCK_HELD() or
similar primitives.  (And one of the reasons for the renaming

Of course, in the case of radix tree, it would be necessary to pass the
boolean in through the radix-tree read-side APIs, which would perhaps be
a bit annoying.

							Thanx, Paul
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