Messages in this thread | | | Date | Mon, 4 Jan 2010 18:19:19 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 2/3] rcu: add debug check for too many rcu_read_unlock() |
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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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