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

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Message-Id: <20120419114811.9e77accb.akpm@linux-foundation.org>
Date:	Thu, 19 Apr 2012 11:48:11 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Roland Dreier <roland@...nel.org>
Cc:	Joerg Roedel <joerg.roedel@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-debug: Fix deadlock with netconsole or other
 drivers that use the DMA API

On Thu, 19 Apr 2012 11:12:53 -0700
Roland Dreier <roland@...nel.org> wrote:

> From: Roland Dreier <roland@...estorage.com>
> 
> If we exhaust the free_entries list, then we print the error message
> 
>     DMA-API: debugging out of memory - disabling
> 
> to the kernel log, while holding free_entries_lock.  Unfortunately, if
> the console driver ends up calling back into the DMA API to map a
> buffer, as eg a NIC driver is quite likely to for the packet netconsole
> asks it to send, this will deadlock on free_entries_lock.
> 
> A fix is pretty simple: if we flip the order of setting global_disable
> to be before we print the error message, then the nested call into the
> DMA API will bail out before trying to get free_entries_lock.
> 
> Signed-off-by: Roland Dreier <roland@...estorage.com>
> ---
>  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?
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