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    SubjectRe: [PATCH tip/core/rcu 2/3] rcu: add debug check for too many rcu_read_unlock()
    On Mon, Jan 04, 2010 at 06:03:08PM -0800, Josh Triplett wrote:
    > On Mon, Jan 04, 2010 at 04:04:01PM -0800, Paul E. McKenney wrote:
    > > From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > >
    > > TREE_PREEMPT_RCU maintains an rcu_read_lock_nesting counter in the
    > > task structure, which happens to be a signed int. So this patch adds a
    > > check for this counter being negative at the end of __rcu_read_unlock().
    > > This check is under CONFIG_PROVE_LOCKING, so can be thought of as being
    > > part of lockdep.
    > >
    > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > > ---
    > > kernel/rcutree_plugin.h | 3 +++
    > > 1 files changed, 3 insertions(+), 0 deletions(-)
    > >
    > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
    > > index f11ebd4..e77cdf3 100644
    > > --- a/kernel/rcutree_plugin.h
    > > +++ b/kernel/rcutree_plugin.h
    > > @@ -304,6 +304,9 @@ void __rcu_read_unlock(void)
    > > if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
    > > unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
    > > rcu_read_unlock_special(t);
    > > +#ifdef CONFIG_PROVE_LOCKING
    > > + WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
    > > +#endif /* #ifdef CONFIG_PROVE_LOCKING */
    > > }
    > > EXPORT_SYMBOL_GPL(__rcu_read_unlock);
    >
    > Given that you *already* need to access t->rcu_read_lock_nesting here,
    > why not just do the test all the time? Ideally you could access
    > t->rcu_read_lock_nesting once, decrement it, and test for both 0 and
    > negative.

    Because I was paranoid about the extra branch. Perhaps needlessly
    paranoid, but this is rcu_read_unlock() we are talking about here. ;-)

    You seem to be suggesting making the first test be "<=", then
    sorting things out later, but given that both the equals-zero and the
    greater-than-zero cases are quite common, I couldn't figure out how to
    avoid the extra test and branch in the common case. Hence the #ifdef.

    Thanx, Paul


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