Re: [PATCH v2] dmaengine: plx_dma: Fix potential deadlock on &plxdev->ring_lock

[Logan Gunthorpe]

On 7/29/23 11:59, Chengfeng Ye wrote:
> As plx_dma_process_desc() is invoked by both tasklet plx_dma_desc_task()
> under softirq context and plx_dma_tx_status() callback that executed under
> process context, the lock aquicision of &plxdev->ring_lock inside
> plx_dma_process_desc() should disable irq otherwise deadlock could happen
> if the irq preempts the execution of process context code while the lock
> is held in process context on the same CPU.
> 
> Possible deadlock scenario:
> plx_dma_tx_status()
>     -> plx_dma_process_desc()
>     -> spin_lock(&plxdev->ring_lock)
>         <tasklet softirq>
>         -> plx_dma_desc_task()
>         -> plx_dma_process_desc()
>         -> spin_lock(&plxdev->ring_lock) (deadlock here)
> 
> This flaw was found by an experimental static analysis tool I am developing
> for irq-related deadlock.
> 
> The lock was changed from spin_lock_bh() to spin_lock() by a previous patch
> for performance concern but unintentionally brought this potential deadlock
> problem.
> 
> This patch reverts back to spin_lock_bh() to fix the deadlock problem.
> 
> Fixes: 1d05a0bdb420 ("dmaengine: plx_dma: Move spin_lock_bh() to spin_lock()")
> Signed-off-by: Chengfeng Ye <dg573847474@xxxxxxxxx>
> 

Reviewed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>

Thanks!

Logan