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

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Date:	Wed, 17 Jun 2009 16:10:18 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Joerg Roedel <joerg.roedel@....com>,
	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


* 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().

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

+1.

	Ingo
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