linux-kernel - Re: [RFC v2 0/6] Track RCU dereferences in RCU read-side critical sections

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Message-ID: <20160225153724.GL3522@linux.vnet.ibm.com>
Date:	Thu, 25 Feb 2016 07:37:24 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>, sasha.levin@...cle.com
Subject: Re: [RFC v2 0/6] Track RCU dereferences in RCU read-side critical
 sections

On Thu, Feb 25, 2016 at 03:32:43PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 16, 2016 at 01:57:39PM +0800, Boqun Feng wrote:
> > As a characteristic of RCU, read-side critical sections have a very
> > loose connection with rcu_dereference()s, which is you can only be sure
> > about an rcu_dereference() might be called in some read-side critical
> > section, but if code gets complex, you may not be sure which read-side
> > critical section exactly, this might be also an problem for some other
> > locking mechanisms, that is the critical sections protecting data and
> > the data accesses protected are not clearly correlated.
> > 
> > In this series, we are introducing LOCKED_ACCESS framework and based on
> > which, we implement the RCU_LOCKED_ACCESS functionality to give us a
> > clear hint: which rcu_dereference() happens in which RCU read-side
> > critical section. 
> > 
> > After this series applied, and if CONFIG_RCU_LOCKED_ACCESS=y, the proc
> > file /proc/locked_access/rcu will show all relationships collected so
> > far for rcu_read_lock() and their friends and rcu_dereference*().
> 
> But why !? What does this bring us, why do I want to even look at these
> patches?

There were some complaints about the difficulty of figuring out what
was being protected by a given rcu_read_lock() in cases where the
corresponding rcu_dereference() is several function calls down, and
especially in cases where the function calls are via pointers.
These cases show up in a number of places, perhaps most prominently
in networking.

Boqun's patches therefore use lockdep to make an association between
each rcu_dereference() and the rcu_read_lock() protecting it.

Seem reasonable, or were the complaints just a flash in the pan?

							Thanx, Paul