lkml.org 
[lkml]   [2012]   [Apr]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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@kernel.org> wrote:

> From: Roland Dreier <roland@purestorage.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@purestorage.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?


\
 
 \ /
  Last update: 2012-04-19 20:51    [W:0.064 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site