Re: [PATCH] iommu/iova: wait 'fq_timer' handler to finish before destroying 'fq'

[Robin Murphy <robin.murphy@arm.com>]

On 2021-12-09 13:17, Robin Murphy wrote:
> Sorry I missed this before...
> 
> On 2019-07-27 10:21, Xiongfeng Wang wrote:
>> Fix following crash that occurs when 'fq_flush_timeout()' access
>> 'fq->lock' while 'iovad->fq' has been cleared. This happens when the
>> 'fq_timer' handler is being executed and we call
>> 'free_iova_flush_queue()'. When the timer handler is being executed,
>> its pending state is cleared and it is detached. This patch use
>> 'del_timer_sync()' to wait for the timer handler 'fq_flush_timeout()' to
>> finish before destroying the flush queue.
> 
> So if I understand correctly, you shut down the device - which naturally 
> frees some DMA mappings into the FQ - then hotplug it out, such that 
> tearing down its group and default domain can end up racing with the 
> timeout firing on a different CPU? It would help if the commit message 
> actually explained that - I've just reverse-engineered it from the given 
> symptom - rather than focusing on details that aren't really important. 
> fq->lock is hardly significant, since *any* access to the FQ while it's 
> being destroyed is fundamentally unsound. I also spent way too long 
> trying to understand the significance of the full stack trace below 
> before realising that it is in fact just irrelevant - there's only one 
> way fq_flush_timeout() ever gets called, and it's the obvious one.
> 
> The fix itself seems reasonable - the kerneldoc for del_timer_sync() is 
> slightly scary, but since free_iova_flush_queue() doesn't touch any of 
> the locks and definitely shouldn't run in IRQ context I believe we're OK.
> 
> This will affect my IOVA refactoring series a little, so I'm happy to 
> help improve the writeup if you like - provided that my understanding is 
> actually correct - and include it in a v2 of that.

FWIW, this is what I came up with:

https://gitlab.arm.com/linux-arm/linux-rm/-/commit/ecea6835baca75b945bd8ecfaa636ff01dabcc1d

Let me know what you think.

Thanks,
Robin.