Messages in this thread | | | Date | Mon, 6 Jun 2011 17:42:50 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [1/4] rcu: Detect uses of rcu read side in extended quiescent states |
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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.
> 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?
Thanx, Paul
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