Re: Deadlock with CMA and CPU hotplug

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hello,

On 2014-10-22 18:57, Laura Abbott wrote:
> We've run into a AB/BA deadlock situation involving a driver lock and
> the CPU hotplug lock on a 3.10 based kernel. The situation is this:
>
> CPU 0                CPU 1
> -----                ----
> Start CPU hotplug
> mutex_lock(&cpu_hotplug.lock)
> Run CPU hotplug notifier
>                 data for driver comes in
>                 mutex_lock(&driver_lock)
>                 driver calls dma_alloc_coherent
>                 alloc_contig_range
>                 lru_add_drain_all
>                 get_online_cpus()
>                 mutex_lock(&cpu_hotplug.lock)
>
> Driver hotplug notifier runs
> mutex_lock(&driver_lock)
>
> The driver itself is out of tree right now[1] and we're looking at
> ways to rework the driver. The best option for rework right now
> though might result in some performance penalties. The size that's
> being allocated can't easily be converted to an atomic allocation either
> It seems like this might be a limitation of where CMA/
> dma_alloc_coherent could potentially be used and make drivers
> unnecessarily aware of CPU hotplug locking.
>
> Does this seem like an actual problem that needs to be fixed or
> is trying to use CMA in a CPU hotplug notifier path just asking
> for trouble?

IMHO doing any allocation without GFP_ATOMIC from a notifier is asking
for problems. I always considered notifiers as callbacks that might be 
called
directly from i.e. interrupts. I don't know much about your code, but 
maybe it
would be possible to move the problematic code from a notifier to a separate
worker or thread?

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

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