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SubjectRe: [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@kernel.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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