linux-kernel - Re: [PATCH 07/10] module: _

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Date:	Thu, 25 Feb 2010 19:10:34 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Alexey Dobriyan <adobriyan@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [PATCH 07/10] module: __rcu annotations

On Thursday 25 February 2010, Paul E. McKenney wrote:
> > 
> > The nice thing about this is that we don't end up with the API explosion
> > for the RCU list primitives.  However, it does require that a given
> > rcu_list_head have a single synchronization-design rule for all uses.
> > Of course, if there were multiple rules, one could construct a check
> > that was simply the union of all the rules, but that would miss some
> > types of errors.

What would it miss? E.g. having the module code check for
(mutex_is_locked(&module_lock) || rcu_read_lock_held) should
cover all cases as far as I can tell.

> > Of course, if this became a problem, there could be an argument to the
> > ->check function that the normal list_for_each_entry_rcu() defaults to
> > "no effect".

I've also been thinking about adding a list_for_each_entry_norcu()
macro that takes an rcu_list_head but then just performs a simple
list_for_each_entry().

> > Or is there a better way to handle this?
> 
> One approach would be to use your original sparse-based approach, but
> use an rcu_deference_const(ptr,lockdep_condition) for cases when the
> value cannot change, for example, when the update-side lock is held.
> This should eliminate most of the false positives, in particular,
> eliminate the need for otherwise-useless rcu_read_lock()s  -- and also
> for the compiler constraints in the normal rcu_dereference().

Right.

> Your pointer-to-function idea could be a really cool way to handle the
> tree algorithms that can be protected either by RCU or by locking.
> The tree nodes could have the pointer to check function, and the
> current rcu_dereference_raw() calls could be replaced by an invocation
> of rcu_dereference_check() that calls the check function.  A check
> function for an RCU-protected tree would use "rcu_read_lock_held() ||
> lockdep_is_held(&update_side_lock)", while a lock-protected tree would
> just use "lockdep_is_held(&update_side_lock)".

I've postponed that problem for now, and updated my series to split
the rculist annotations from the basic __rcu pointer annotations,
as well as to apply on top of your patches in tip/core/rcu,
see http://git.kernel.org/?p=linux/kernel/git/arnd/playground.git;\
a=shortlog;h=refs/heads/rcu-annotate-tip.

Should we merge the simple annotations in this merge window and
then think about rculist and trees separately?

	Arnd
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