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

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Date:	Tue, 16 Jun 2009 13:39:13 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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 Tue, Jun 16, 2009 at 12:36:06PM +0200, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@...e.hu> wrote:
> 
> > This is something we noticed recently: dma-debug uses a lot of 
> > lock classes and thus creates a really large lock-graph, depleting 
> > the reserves quickly.
> 
> yep, i think dma-debug is the culprit. It has:
> 
>  #define HASH_SIZE       1024ULL
> 
> each hash entry has a separate lock.
> 
> But this should be easily to solve: a special lock-class for these 
> locks. Patch below (warning: utterly untested!).
> 
> (Btw., that SPIN_LOCK_UNLOCKED init method should really be 
> deprecated for real.)
> 
> 	Ingo
> 
> diff --git a/lib/dma-debug.c b/lib/dma-debug.c
> index 3b93129..012c240 100644
> --- a/lib/dma-debug.c
> +++ 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) {

Ok, cool. I would apply it but I don't know enough about lockdep
internals to write a meaningful commit message. Can you please provide
one so I can commit this?

	Joerg

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