linux-kernel - Re: [PATCH] dma-debug: Fix deadlock with netconsole or other drivers that use the DMA API

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Date:	Fri, 20 Apr 2012 13:22:06 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Roland Dreier <roland@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma-debug: Fix deadlock with netconsole or other drivers
 that use the DMA API

On Thu, Apr 19, 2012 at 11:48:11AM -0700, Andrew Morton wrote:
> On Thu, 19 Apr 2012 11:12:53 -0700
> Roland Dreier <roland@...nel.org> wrote:
> > ---
> >  lib/dma-debug.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/lib/dma-debug.c b/lib/dma-debug.c
> > index 13ef233..f198b4e 100644
> > --- a/lib/dma-debug.c
> > +++ b/lib/dma-debug.c
> > @@ -436,8 +436,8 @@ static struct dma_debug_entry *dma_entry_alloc(void)
> >  	spin_lock_irqsave(&free_entries_lock, flags);
> >  
> >  	if (list_empty(&free_entries)) {
> > -		pr_err("DMA-API: debugging out of memory - disabling\n");
> >  		global_disable = true;
> > +		pr_err("DMA-API: debugging out of memory - disabling\n");
> >  		goto out;
> >  	}
> 
> So *any* printk can deadlock if free_entries_lock is held and
> global_disable==false?
> 
> In that case we're going to need much sterner fixes.  Any list_head
> operation can do a printk if list_head debugging is enabled. 
> dma_debug_resize_entries() does a kfree() under free_entries_lock(!).
> 
> Methinks we need a more general fix?

Hmm, I think the best way to fix it is to switch the hash-bucket lists
to rcu. Then the lock is only needed in hash_bucket_add/del and we
should be fine.
But that is probably only a fix for the next merge window.


	Joerg

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