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SubjectRe: [PATCH] dmaengine: pl330: fix double lock
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Hi Iago,

On 2017-01-11 14:00, iari@itu.dk wrote:
> From: Iago Abal <mail@iagoabal.eu>
>
> 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@iagoabal.eu>

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

> ---
> 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

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