Date:	Tue, 08 Mar 2016 10:40:11 +0000
From:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Robert Baldyga <r.baldyga@...sung.com>,
	Lukasz Czerwinski <l.czerwinski@...sung.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Jaswinder Singh <jassisinghbrar@...il.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] dmaengine: pl330: Fix some race conditions in residue
 calculation

On Tue, 2016-03-08 at 09:42 +0530, Vinod Koul wrote:
> On Wed, Feb 24, 2016 at 01:14:34PM +0000, Jon Medhurst (Tixy) wrote:
> > The residue calculation in pl330_tx_status doesn't handle transitional
> > states that occur at the time one descriptor (A) is completed and the
> > next (B) is started. Specifically, both A and B can simultaneously be in
> > the BUSY state and at this time the thread's 'req_running' may (or may
> > not) be -1.
> 
> you are under lock so descriptor state wont be update while we are it.
> 
> Also the query for residue is for "a descriptor" not whatever is the current
> running descriptor...
> 
> > 
> > To cope with this situation we change the code to ensure A is treated as
> > complete and B as having not yet started. Prior to the change, the code
> > would calculate a transferred byte count as if both A and B had
> > completed.
> 
> You query for either A or B not both!

I've probably been using wrong/ambiguous terminology...

In my description I'm using 'descriptor' to refer to a 'struct
dma_pl330_desc', I guess other people assume 'struct
dma_async_tx_descriptor'?

The situation I was debugging was audio playback, where ASoC ends up
calling pl330_prep_dma_cyclic() with a period one quarter the length of
the buffer it is using, so that results in four dma_pl330_desc
'descriptors' being created to cover that buffer. These later get
submitted to a DMA channel (struct dma_pl330_chan) which has a list of
these that it is processing (the 'work_list').

The residual calculation that currently exists in pl08x_dma_tx_status()
is iterating this work_list and summing the length of currently
transferring 'descriptor' with those later pending ones. I believe that
is correct behaviour because these 'descriptors' (dma_pl330_desc) are
all internal implementation details of the driver, and the dmaengine
API's are dealing in units of 'dma_async_tx_descriptor' ?

If the current code is OK in this regard, it is definitely buggy because
it doesn't cope with the situation when two dma_pl330_desc's are in the
state 'BUSY' a, which I have seen occur when debugging this issue, had
worked out can happen by analysing the code, and is acknowledged by the
in-source comments for enum desc_status...

	/*
	 * Sitting on the work_list and already submitted
	 * to the PL330 core. Not more than two descriptors
	 * of a channel can be BUSY at any time.
	 */
	BUSY,

In my problematic usecase I have userside code calling ALSA ioctls to
poll the current audio playback position which results in
pl08x_dma_tx_status() being called multiple times a second. After only a
second or two the buggy situation gets hit, resulting in a
miscalculation that ASoC interprets as a buffer underflow and so it
stops the stream.

I spent several days debugging this, with enough ad hoc tests and
printk's littered everywhere to be very confident as to how things are
going wrong - what I'm not not totally confident of is how things should
be properly fixed.

This patch appears to fix the situation that I was hitting, but it
really looks like there isn't any locking that prevent this polling use
of pl08x_dma_tx_status() from happening concurrently with the irq
handler reprogramming the hardware for the next dma_pl330_desc. I didn't
attempt any fix for that for fear of introducing bugs in what looks like
complex code, and because it's not a problem I saw happen in practice.

-- Tixy

> 
> > 
> > Fixes: aee4d1fac887 ("dmaengine: pl330: improve pl330_tx_status() function")
> > 
> > Signed-off-by: Jon Medhurst <tixy@...aro.org>
> > ---
> > 
> > I discovered this issue when trying to work out why audio stopped
> > working on ARM's Juno platform and bisected it to commit aee4d1fac887.
> > Whilst this patch seems to fix the problems I was seeing, I can't help
> > but think there are more race conditions with this code. E.g. if the
> > running descriptor changes under us, pl330_get_current_xferred_count
> > can end up reading values from hardware that relate to a different
> > descriptor. And if we're really unlucky, the reading of the 'val' and
> > 'addr' values in pl330_get_current_xferred_count can come from different
> > descriptors. I don't know if there is any locks we can use to prevent
> > such races or if we need to try and detect when things have changed and
> > redo/abort the residue calculation...
> > 
> >  drivers/dma/pl330.c | 24 ++++++++++++++++++++----
> >  1 file changed, 20 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
> > index 17ee758..55e3c5f 100644
> > --- a/drivers/dma/pl330.c
> > +++ b/drivers/dma/pl330.c
> > @@ -2240,6 +2240,7 @@ pl330_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
> >  	struct dma_pl330_desc *desc, *running = NULL;
> >  	struct dma_pl330_chan *pch = to_pchan(chan);
> >  	unsigned int transferred, residual = 0;
> > +	bool first_busy;
> >  
> >  	ret = dma_cookie_status(chan, cookie, txstate);
> >  
> > @@ -2253,16 +2254,31 @@ pl330_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
> >  
> >  	if (pch->thread->req_running != -1)
> >  		running = pch->thread->req[pch->thread->req_running].desc;
> > +	first_busy = true;
> >  
> >  	/* Check in pending list */
> >  	list_for_each_entry(desc, &pch->work_list, node) {
> >  		if (desc->status == DONE)
> >  			transferred = desc->bytes_requested;
> > -		else if (running && desc == running)
> > -			transferred =
> > -				pl330_get_current_xferred_count(pch, desc);
> > -		else
> > +		else if (desc->status == BUSY && first_busy) {
> > +			first_busy = false;
> > +			if (running && desc == running) {
> > +				transferred =
> > +					pl330_get_current_xferred_count(pch, desc);
> > +			} else {
> > +				/* BUSY but not running means it's just completed */
> > +				transferred = desc->bytes_requested;
> > +			}
> > +		} else {
> > +			/*
> > +			 * Descriptor is either in PREP state queued for future
> > +			 * transfer or it is the second BUSY descriptor we have
> > +			 * seen. The latter case means it has just, or is about
> > +			 * to be, started, so treat it as having not yet
> > +			 * transferred any bytes, the same as PREP.
> > +			 */
> >  			transferred = 0;
> > +		}
> >  		residual += desc->bytes_requested - transferred;
> >  		if (desc->txd.cookie == cookie) {
> >  			switch (desc->status) {
> > -- 
> > 2.1.4
> > 
> > 
> 

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