Messages in this thread | | | Date | Tue, 5 Jan 2010 08:21:37 -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 06:28:15PM -0800, Josh Triplett wrote: > On Mon, Jan 04, 2010 at 06:19:19PM -0800, Paul E. McKenney wrote: > > 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. > > No, I think you could simply read the predecremented value into a local > variable, test it once with == 0, then have the WARN_ON_ONCE, and hope > that the compiler figures out it can just test the register once and > then do multiple jumps on the same flags. > > You could try it and see what code it generates.
I agree that a smart compiler could share condition-code state, but there still will be the extra branch. (Keep in mind that this is a .h file, so #ifdef is permitted -- though I might nevertheless make a one-line function/macro.)
Thanx, Paul
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