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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 19 Apr 2012 16:36:56 -0700
Roland Dreier <roland@kernel.org> wrote:

> On Thu, Apr 19, 2012 at 11:48 AM, Andrew Morton
> <akpm@linux-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.

I suppose one could do something like

static DEFINE_SPINLOCK(lock);
static struct task_struct *owner;
static unsigned depth;

/*
 * Nice comments go here
 */
static unsigned long free_entries_lock(void)
{
	unsigned long flags = 0;

	if (owner == current) {
		depth++;
	} else {
		/* Permit recursive locking */
		spin_lock_irqsave(&lock, flags);
		BUG_ON(depth != 0);
		BUG_ON(owner != NULL);
		owner = current;
	}
	return flags;
}

static void free_entries_unlock(unsigned long flags)
{
	BUG_ON(owner != current);
	if (!--depth) {
		owner = NULL;
		spin_lock_irqrestore(&lock, flags);
	}
}

After removing the bugs, I think that's safe wrt interrupts?