Messages in this thread |  | | | Date | Tue, 7 Jun 2011 20:49:01 +0200 | | From | Frederic Weisbecker <> | | 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 11:34:14AM -0700, Paul E. McKenney wrote: > On Tue, Jun 07, 2011 at 02:58:13PM +0200, Frederic Weisbecker wrote: > > On Mon, Jun 06, 2011 at 09:40:05PM -0700, Paul E. McKenney wrote:> > > > 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. > > > > Ah, in fact it doesn't pass through any lockdep check. > > > > It's only a function called might_sleep() that is placed in functions > > that can sleep. And inside might_sleep() it checks whether it is in a preemptible > > area. So it's actually locking-agnostic, it only relies on the preempt_count > > and some more for the preempt rcu cases. > > > > I think it is called CONFIG_DEBUG_SPINLOCK_SLEEP because it was first used > > for spinlock debugging purposes. But then it has a broader use now: sleep > > inside preemptible section, sleep inside interrupts, sleep inside rcu. > > But the __might_sleep() function can only differentiate between > spinlocks and sleeplocks if CONFIG_PREEMPT=y.
It doesn't differentiate between locks but checks on the lowest level by looking at the preempt count. But yeah it only works if CONFIG_PREEMPT, which is why I proposed to inc/dec the preempt count also when we have that DEBUG_SPINLOCK_SLEEP.
> > > It certainly deserves a rename, like CONFIG_DEBUG_ILLEGAL_SLEEP. > > Hmmm... It already checks for sleeping in the middle of a > preempt_disable() as well as in a spinlock critical section. > So the need for a rename is independent of any RCU checking.
Sure, rcu just adds itself to the pile of users of might_sleep(), thus it would be a nice cleanup to rename the option to something more generic. But that rename is not necessary to improve RCU checking.
> > > > In contrast, with RCU, this is straightforward -- check for rcu_sched > > > and rcu_bh, but not SRCU. > > Actually it makes sense to keep the checks in rcu_note_context_switch(), > as there are places that call schedule() directly without a might_sleep(). > Perhaps having checks in both places is the correct approach?
In this case it makes more sense to add your checks in schedule_debug(), so that we don't wait for a context switch to detect the bug.
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