Re: [PATCH] dmaengine: pl330: fix double lock

[Marek Szyprowski]

Hi Iago,

On 2017-01-11 14:00, iari@xxxxxx wrote:
> From: Iago Abal <mail@xxxxxxxxxxx>
>
> The static bug finder EBA (http://www.iagoabal.eu/eba/) reported the
> following double-lock bug:
>
>      Double lock:
>      1. spin_lock_irqsave(pch->lock, flags) at pl330_free_chan_resources:2236;
>      2. call to function `pl330_release_channel' immediately after;
>      3. call to function `dma_pl330_rqcb' in line 1753;
>      4. spin_lock_irqsave(pch->lock, flags) at dma_pl330_rqcb:1505.
>
> I have fixed it as suggested by Marek Szyprowski.
>
> First, I have replaced `pch->lock' with `pl330->lock' in functions
> `pl330_alloc_chan_resources' and `pl330_free_chan_resources'. This avoids
> the double-lock by acquiring a different lock than `dma_pl330_rqcb'.
>
> NOTE that, as a result, `pl330_free_chan_resources' executes
> `list_splice_tail_init' on `pch->work_list' under lock `pl330->lock',
> whereas in the rest of the code `pch->work_list' is protected by
> `pch->lock'. I don't know if this may cause race conditions. Similarly
> `pch->cyclic' is written by `pl330_alloc_chan_resources' under
> `pl330->lock' but read by `pl330_tx_submit' under `pch->lock'.

At the moment of initialization and releasing resources this is not
a problem.

> Second, I have removed locking from `pl330_request_channel' and
> `pl330_release_channel' functions. Function `pl330_request_channel' is
> only called from `pl330_alloc_chan_resources', so the lock is already
> held. Function `pl330_release_channel' is called from
> `pl330_free_chan_resources', which already holds the lock, and from
> `pl330_del'. Function `pl330_del' is called in an error path of
> `pl330_probe' and at the end of `pl330_remove', but I assume that there
> cannot be concurrent accesses to the protected data at those points.
>
> Signed-off-by: Iago Abal <mail@xxxxxxxxxxx>

Reviewed-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

> ---
>   drivers/dma/pl330.c | 19 ++++++-------------
>   1 file changed, 6 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
> index 87fd015..3370e56 100644
> --- a/drivers/dma/pl330.c
> +++ b/drivers/dma/pl330.c
> @@ -1696,7 +1696,6 @@ static bool _chan_ns(const struct pl330_dmac *pl330, int i)
>   static struct pl330_thread *pl330_request_channel(struct pl330_dmac *pl330)
>   {
>   	struct pl330_thread *thrd = NULL;
> -	unsigned long flags;
>   	int chans, i;
>   
>   	if (pl330->state == DYING)
> @@ -1704,8 +1703,6 @@ static struct pl330_thread *pl330_request_channel(struct pl330_dmac *pl330)
>   
>   	chans = pl330->pcfg.num_chan;
>   
> -	spin_lock_irqsave(&pl330->lock, flags);
> -
>   	for (i = 0; i < chans; i++) {
>   		thrd = &pl330->channels[i];
>   		if ((thrd->free) && (!_manager_ns(thrd) ||
> @@ -1723,8 +1720,6 @@ static struct pl330_thread *pl330_request_channel(struct pl330_dmac *pl330)
>   		thrd = NULL;
>   	}
>   
> -	spin_unlock_irqrestore(&pl330->lock, flags);
> -
>   	return thrd;
>   }
>   
> @@ -1742,7 +1737,6 @@ static inline void _free_event(struct pl330_thread *thrd, int ev)
>   static void pl330_release_channel(struct pl330_thread *thrd)
>   {
>   	struct pl330_dmac *pl330;
> -	unsigned long flags;
>   
>   	if (!thrd || thrd->free)
>   		return;
> @@ -1754,10 +1748,8 @@ static void pl330_release_channel(struct pl330_thread *thrd)
>   
>   	pl330 = thrd->dmac;
>   
> -	spin_lock_irqsave(&pl330->lock, flags);
>   	_free_event(thrd, thrd->ev);
>   	thrd->free = true;
> -	spin_unlock_irqrestore(&pl330->lock, flags);
>   }
>   
>   /* Initialize the structure for PL330 configuration, that can be used
> @@ -2117,20 +2109,20 @@ static int pl330_alloc_chan_resources(struct dma_chan *chan)
>   	struct pl330_dmac *pl330 = pch->dmac;
>   	unsigned long flags;
>   
> -	spin_lock_irqsave(&pch->lock, flags);
> +	spin_lock_irqsave(&pl330->lock, flags);
>   
>   	dma_cookie_init(chan);
>   	pch->cyclic = false;
>   
>   	pch->thread = pl330_request_channel(pl330);
>   	if (!pch->thread) {
> -		spin_unlock_irqrestore(&pch->lock, flags);
> +		spin_unlock_irqrestore(&pl330->lock, flags);
>   		return -ENOMEM;
>   	}
>   
>   	tasklet_init(&pch->task, pl330_tasklet, (unsigned long) pch);
>   
> -	spin_unlock_irqrestore(&pch->lock, flags);
> +	spin_unlock_irqrestore(&pl330->lock, flags);
>   
>   	return 1;
>   }
> @@ -2228,12 +2220,13 @@ static int pl330_pause(struct dma_chan *chan)
>   static void pl330_free_chan_resources(struct dma_chan *chan)
>   {
>   	struct dma_pl330_chan *pch = to_pchan(chan);
> +	struct pl330_dmac *pl330 = pch->dmac;
>   	unsigned long flags;
>   
>   	tasklet_kill(&pch->task);
>   
>   	pm_runtime_get_sync(pch->dmac->ddma.dev);
> -	spin_lock_irqsave(&pch->lock, flags);
> +	spin_lock_irqsave(&pl330->lock, flags);
>   
>   	pl330_release_channel(pch->thread);
>   	pch->thread = NULL;
> @@ -2241,7 +2234,7 @@ static void pl330_free_chan_resources(struct dma_chan *chan)
>   	if (pch->cyclic)
>   		list_splice_tail_init(&pch->work_list, &pch->dmac->desc_pool);
>   
> -	spin_unlock_irqrestore(&pch->lock, flags);
> +	spin_unlock_irqrestore(&pl330->lock, flags);
>   	pm_runtime_mark_last_busy(pch->dmac->ddma.dev);
>   	pm_runtime_put_autosuspend(pch->dmac->ddma.dev);
>   }

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland