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    SubjectRe: [1/4] rcu: Detect uses of rcu read side in extended quiescent states
    On Mon, Jun 06, 2011 at 05:42:50PM -0700, Paul E. McKenney wrote:
    > On Tue, Jun 07, 2011 at 02:19:07AM +0200, Frederic Weisbecker wrote:
    > > On Mon, Jun 06, 2011 at 11:10:21AM -0700, Paul E. McKenney wrote:
    > > > commit c15d76f26712bd5228aa0c6af7a7e7c492a812c9
    > > > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > > > Date: Tue May 24 08:31:09 2011 -0700
    > > >
    > > > rcu: Restore checks for blocking in RCU read-side critical sections
    > > >
    > > > 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>
    > >
    > > A little comment about this patch:
    > >
    > > <snip>
    > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > > > index 88547c8..8b4b3da 100644
    > > > --- a/kernel/rcutree.c
    > > > +++ b/kernel/rcutree.c
    > > > @@ -153,6 +153,12 @@ void rcu_bh_qs(int cpu)
    > > > */
    > > > void rcu_note_context_switch(int cpu)
    > > > {
    > > > + rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),
    > > > + "Illegal context switch in RCU-bh"
    > > > + " read-side critical section");
    > > > + rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
    > > > + "Illegal context switch in RCU-sched"
    > > > + " read-side critical section");
    > >
    > > This looks like more a check to make inside might_sleep().
    > > It's better because might_sleep() triggers the check even if
    > > we don't actually go to sleep.
    >
    > This does make quite a bit of sense.
    >
    > > In fact I believe might_sleep() already does the job fine:
    > >
    > > If !PREEMPT, might_sleep() detects that preemption is disabled
    > > by rcu_read_lock().
    >
    > If !PREEMPT, isn't the preempt_disable() called by rcu_read_lock()
    > implemented as follows?
    >
    > #define preempt_disable() do { } while (0)
    >
    > Unless I am missing something, __might_sleep() won't detect that.

    Ah, right.

    > > If PREEMPT, might_sleep() checks rcu_preempt_depth().
    >
    > Agreed, for CONFIG_TREE_PREEMPT_RCU and CONFIG_TINY_PREEMPT_RCU,
    > the existing might_sleep() checks do cover it.
    >
    > So I could export an rcu_might_sleep() or some such that contained
    > the above two rcu_lockdep_assert()s, but only if !PREEMPT_RCU.
    > If PREEMPT_RCU, rcu_might_sleep() would be a no-op.
    >
    > Seem reasonable, or am I missing something?

    Ok but that only improves the rcu debugging. What about instead improving
    might_sleep() to also work in !PREEMPT, so that it profits to any detection
    of forbidden sleeping (sleep inside spinlock, preempt_disable, might_fault, etc...)

    We could define a new config:

    config PREEMPT_COUNT
    default PREEMPT || DEBUG_SPINLOCK_SLEEP

    and build preempt_disable()/preempt_enable() on top of that instead
    of using CONFIG_PREEMPT directly.

    Does that look sane?


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