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

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Message-ID: <CAL1RGDXQ9NPKaNxBcNAYKknpSSScwXVa+gr8jXOFFU4ZjjSJ=A@mail.gmail.com>
Date:	Thu, 19 Apr 2012 16:36:56 -0700
From:	Roland Dreier <roland@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.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, Apr 19, 2012 at 11:48 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> So *any* printk can deadlock if free_entries_lock is held and
> global_disable==false?

apparently.

> 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?

sigh... no good deed goes unpunished.

OK, will look at it.  Just to make things even more fun, all the
err_printk() stuff can potentially deadlock on the hash bucket
lock, although that requires enough bad luck a collision to happen.

 - R.
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