linux-kernel - Re: bug in tty ldisc and friends

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Date:	Wed, 17 Jun 2009 16:28:19 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: bug in tty ldisc and friends

On Wed, Jun 17, 2009 at 04:10:18PM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > On Tue, 2009-06-16 at 12:36 +0200, Ingo Molnar wrote:
> > 
> > > +++ b/lib/dma-debug.c
> > > @@ -62,6 +62,8 @@ struct dma_debug_entry {
> > >  #endif
> > >  };
> > >  
> > > +static struct lock_class_key hash_bucket_class;
> > > +
> > >  struct hash_bucket {
> > >  	struct list_head list;
> > >  	spinlock_t lock;
> > > @@ -716,7 +718,8 @@ void dma_debug_init(u32 num_entries)
> > >  
> > >  	for (i = 0; i < HASH_SIZE; ++i) {
> > >  		INIT_LIST_HEAD(&dma_entry_hash[i].list);
> > > -		dma_entry_hash[i].lock = SPIN_LOCK_UNLOCKED;
> > > +		spin_lock_init(&dma_entry_hash[i].lock);
> > > +		lockdep_set_lock_class(&dma_entry_hash[i].lock, &hash_bucket_class);
> > >  	}
> > >  
> > >  	if (dma_debug_fs_init() != 0) {
> > 
> > I don't see what you need that lockdep_set_class() for, the 
> > spin_lock_init() would already set a class specific to the 
> > callsite. And since all these buckets are initialized from the 
> > same place (ie, this loop), they'd all share the same class.
> >
> > Its just that the old style SPIN_LOCK_UNLOCKED doesn't set a class 
> > and then reverts to the static address of the lock object itself 
> > that these locks used to have different classes.
> > 
> > So in short, yes the above patch should fix it, but it could be 
> > done shorter..
> 
> Indeed ... Joerg: mind removing the set_class complication and just 
> switch it to spin_lock_init().

Ok, done.

> 
> > we really should do another round of SPIN_LOCK_UNLOCKED cleanups 
> > and finally remove that thing.
> 
> +1.

A first step might be updating the documentation under
Documentation/spinlocks.txt to teach people the new right way of
initializing spinlocks :-)

	Joerg

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