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    SubjectRe: [1/4] rcu: Detect uses of rcu read side in extended quiescent states
    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

    > > 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.

     \ /
      Last update: 2011-06-07 20:51    [W:0.022 / U:6.912 seconds]
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