Messages in this thread | | | Date | Mon, 6 Jun 2011 21:40:05 -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 03:36:32AM +0200, Frederic Weisbecker wrote: > 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?
The bit I am missing is how to distinguish between spinlocks (where sleeping is illegal) and mutexes (where sleeping is perfectly fine). We could teach lockdep the difference, I suppose, but it is not clear to me that it is worth it.
In contrast, with RCU, this is straightforward -- check for rcu_sched and rcu_bh, but not SRCU.
Thanx, Paul
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