linux-kernel - Re: [PATCH tip/core/rcu 04/28] rcu: Restore checks for blocking in RCU read-side critical sections

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Message-ID: <20110608232833.GD25771@somewhere.redhat.com>
Date:	Thu, 9 Jun 2011 01:28:35 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 04/28] rcu: Restore checks for blocking in
 RCU read-side critical sections

On Wed, Jun 08, 2011 at 12:29:43PM -0700, Paul E. McKenney wrote:
> Long ago, using TREE_RCU with PREEMPT would result in "scheduling
> while atomic" diagnostics if you blocked in an RCU read-side critical
> section.  However, PREEMPT now implies TREE_PREEMPT_RCU, which defeats
> this diagnostic.  This commit therefore adds a replacement diagnostic
> based on PROVE_RCU.
> 
> Because rcu_lockdep_assert() and lockdep_rcu_dereference() are now being
> used for things that have nothing to do with rcu_dereference(), rename
> lockdep_rcu_dereference() to lockdep_rcu_suspicious() and add a third
> argument that is a string indicating what is suspicious.  This third
> argument is passed in from a new third argument to rcu_lockdep_assert().
> Update all calls to rcu_lockdep_assert() to add an informative third
> argument.
> 
> Finally, add a pair of rcu_lockdep_assert() calls from within
> rcu_note_context_switch(), one complaining if a context switch occurs
> in an RCU-bh read-side critical section and another complaining if a
> context switch occurs in an RCU-sched read-side critical section.
> These are present only if the PROVE_RCU kernel parameter is enabled.
> 
> Again, you must enable PROVE_RCU to see these new diagnostics.  But you
> are enabling PROVE_RCU to check out new RCU uses in any case, aren't you?
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

So, do you think we can get rid of this patch now that we are going to have CONFIG_ATOMIC_SLEEP
working everywhere?

The last remaining piece we need is to check rcu_preempt_depth() from schedule_debug(),
which does a kind of lightweight might_sleep() check alike.
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