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    SubjectRe: [PATCH tip/core/rcu 04/28] rcu: Restore checks for blocking in RCU read-side critical sections
    On Wed, Jun 08, 2011 at 04:46:22PM -0700, Paul E. McKenney wrote:
    > On Thu, Jun 09, 2011 at 01:28:35AM +0200, Frederic Weisbecker wrote:
    > > 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@linux.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.
    >
    > I believe that we need them both. Your patch provides a lightweight
    > check. Mine is way heavier weight (CONFIG_PROVE_RCU is required), but
    > tells you in what function the offending RCU read-side critical section
    > was entered.

    Well, that can be found easily in the stacktrace. But ok.


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