Re: dmatest regression in 3.10-rc1

From: Vinod Koul
Date: Fri May 17 2013 - 09:10:29 EST

On Thu, May 16, 2013 at 04:35:53PM +0100, Will Deacon wrote:
> On Wed, May 15, 2013 at 04:28:03PM +0100, Will Deacon wrote:
> > I've been observing a regression in the dmatest module with 3.10-rc1. It
> > manifests as either:
> >
> > - a spurious timeout on one or more of the channel threads
> > - a complete kernel lockup (loss of console)
> > - a panic (see below, noting that the callback [dmatest_callback] is
> > dereferencing a NULL pointer)
> >
> > If I revert 77101ce578bb ("dmatest: cancel thread immediately when asked
> > for") then things are rosy again, but I'm not sure if this is hiding another
> > problem.
> Right, so I think I understand what's causing this, but I'll leave it to
> Andriy to suggest a fix. The problem comes about because the dmatest
> module is now driven from debugfs, making it possible to unload the module
> whilst a test run is in progress. In this case:
> - The DMA threads will return from wait_event_freezable_timeout(...)
> due to kthread_should_stop() returning true, and subsequently
> report failure because done.done is false.
> - The DMA engines may not be idle, so the asynchronous callback can
> be invoked after we've started cleaning up, explaining the NULL
> dereference I'm seeing.
> The solutions are either fixing the module exit code to cope with concurrent
> DMA transfers or to revert 77101ce578bb and not allow the channel threads to
> return mid-transfer.
We need to properly abort the channels on removal. This is already handled in
the code but the kthread_stop is called after the transactions are aborted. It
should be the other way round. Can you try with below patch


diff --git a/drivers/dma/dmatest.c b/drivers/dma/dmatest.c
index d8ce4ec..4e8d581 100644
--- a/drivers/dma/dmatest.c
+++ b/drivers/dma/dmatest.c
@@ -822,6 +822,9 @@ static void dmatest_cleanup_channel(struct dmatest_chan *dtc)
struct dmatest_thread *_thread;
int ret;

+ /* terminate all transfers on specified channels */
+ dmaengine_terminate_all(dtc->chan);
list_for_each_entry_safe(thread, _thread, &dtc->threads, node) {
ret = kthread_stop(thread->task);
pr_debug("dmatest: thread %s exited with status %d\n",
@@ -830,9 +833,6 @@ static void dmatest_cleanup_channel(struct dmatest_chan *dtc)

- /* terminate all transfers on specified channels */
- dmaengine_terminate_all(dtc->chan);

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