Re: [PATCH] dmaengine: xgene: Fix potential deadlock on &chan->lock

[Christophe JAILLET]

Le 26/07/2023 à 13:16, Chengfeng Ye a écrit :
> As xgene_dma_cleanup_descriptors() is invoked by both tasklet
> xgene_dma_tasklet_cb() under softirq context and
> xgene_dma_free_chan_resources() callback that executed under process
> context, the lock aquicision of &chan->lock inside
> xgene_dma_cleanup_descriptors() should disable irq otherwise deadlock
> could happen if the tasklet softirq preempts the execution of process
> context code while the lock is held in process context on the same CPU.
>
> Possible deadlock scenario:
> xgene_dma_free_chan_resources()
>      -> xgene_dma_cleanup_descriptors()
>      -> spin_lock(&chan->lock)
>          <tasklet softirq>
>          -> xgene_dma_tasklet_cb()
>          -> xgene_dma_cleanup_descriptors()
>          -> spin_lock(&chan->lock) (deadlock here)
>
> This flaw was found by an experimental static analysis tool I am developing
> for irq-related deadlock.

Hi,

first of all, for what I've seen from your numerous recent patches, all 
this look real great ! :)
And your experimental tool looks really promising.


Even if I'm not always confident with my understanding of locking and 
related subtilities, I wonder if in the cases of <tasklet softirq>, like 
above, using spin_lock_bh() would be enough?
It should be less agressive than spin_lock_irqsave() but still handle 
the use case you have spotted.


Just my 2c.

CJ


> The tentative patch fixes the potential deadlock by spin_lock_irqsave() in
> plx_dma_process_desc() to disable irq while lock is held.
>
> Signed-off-by: Chengfeng Ye <dg573847474@xxxxxxxxx>
> ---
>   drivers/dma/xgene-dma.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma/xgene-dma.c b/drivers/dma/xgene-dma.c
> index 3589b4ef50b8..e766511badcf 100644
> --- a/drivers/dma/xgene-dma.c
> +++ b/drivers/dma/xgene-dma.c
> @@ -689,11 +689,12 @@ static void xgene_dma_cleanup_descriptors(struct xgene_dma_chan *chan)
>   	struct xgene_dma_desc_sw *desc_sw, *_desc_sw;
>   	struct xgene_dma_desc_hw *desc_hw;
>   	struct list_head ld_completed;
> +	unsigned long flags;
>   	u8 status;
>   
>   	INIT_LIST_HEAD(&ld_completed);
>   
> -	spin_lock(&chan->lock);
> +	spin_lock_irqsave(&chan->lock, flags);
>   
>   	/* Clean already completed and acked descriptors */
>   	xgene_dma_clean_completed_descriptor(chan);
> @@ -762,7 +763,7 @@ static void xgene_dma_cleanup_descriptors(struct xgene_dma_chan *chan)
>   	 */
>   	xgene_chan_xfer_ld_pending(chan);
>   
> -	spin_unlock(&chan->lock);
> +	spin_unlock_irqrestore(&chan->lock, flags);
>   
>   	/* Run the callback for each descriptor, in order */
>   	list_for_each_entry_safe(desc_sw, _desc_sw, &ld_completed, node) {