linux-kernel - Re: irda rmmod lockdep trace.

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Date:	Fri, 16 Mar 2007 22:19:00 +0200
From:	Samuel Ortiz <samuel@...tiz.org>
To:	David Miller <davem@...emloft.net>
Cc:	davej@...hat.com, linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: irda rmmod lockdep trace.

On Tue, Mar 13, 2007 at 07:22:37PM -0700, David Miller wrote:
> From: Samuel Ortiz <samuel@...tiz.org>
> Date: Wed, 14 Mar 2007 02:50:03 +0200
> 
> > On Mon, Mar 12, 2007 at 04:49:21PM -0700, David Miller wrote:
> > > I would strongly caution against adding any run-time overhead just to
> > > cure a false lockdep warning.  Even adding a new function argument
> > > is too much IMHO.
> > > 
> > > Make the cost show up for lockdep only, perhaps by putting each
> > > hashbin lock into a seperate locking class?
> > Does that look better to you:
> 
> Yes, it does.:)
Unfortunately, it doesn't work, as the lock key is not on the stack. We get
hit by the lockdep code checking if our lock key is static:

      if (!static_obj(key)) {
                printk("BUG: key %p not in .data!\n", key);
                DEBUG_LOCKS_WARN_ON(1);
                return;
        }

So, instead, I propose the following, which does work, and adds runtime
overhead only when LOCKDEP is enabled:

---
 net/irda/irqueue.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/net/irda/irqueue.c b/net/irda/irqueue.c
index 9266233..d058b46 100644
--- a/net/irda/irqueue.c
+++ b/net/irda/irqueue.c
@@ -384,6 +384,9 @@ EXPORT_SYMBOL(hashbin_new);
  *    for deallocating this structure if it's complex. If not the user can
  *    just supply kfree, which should take care of the job.
  */
+#ifdef CONFIG_LOCKDEP
+static int hashbin_lock_depth = 0;
+#endif
 int hashbin_delete( hashbin_t* hashbin, FREE_FUNC free_func)
 {
 	irda_queue_t* queue;
@@ -395,7 +398,8 @@ int hashbin_delete( hashbin_t* hashbin, FREE_FUNC free_func)
 
 	/* Synchronize */
 	if ( hashbin->hb_type & HB_LOCK ) {
-		spin_lock_irqsave(&hashbin->hb_spinlock, flags);
+		spin_lock_irqsave_nested(&hashbin->hb_spinlock, flags,
+					 hashbin_lock_depth++);
 	}
 
 	/*
@@ -419,6 +423,9 @@ int hashbin_delete( hashbin_t* hashbin, FREE_FUNC free_func)
 	/* Release lock */
 	if ( hashbin->hb_type & HB_LOCK) {
 		spin_unlock_irqrestore(&hashbin->hb_spinlock, flags);
+#ifdef CONFIG_LOCKDEP
+		hashbin_lock_depth--;
+#endif
 	}
 
 	/*

-
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