[BUG] Circular locking dependency - DRM/CMA/MM/hotplug/...

[Russell King - ARM Linux]

Please learn how to trim emails down to contain only the bits relevant to
your reply, thanks.

On Tue, Feb 18, 2014 at 03:25:21PM +0100, Marek Szyprowski wrote:
> Hello,
>
> On 2014-02-12 17:33, Russell King - ARM Linux wrote:
>> So, the full locking dependency tree is this:
>>
>> CPU0		CPU1		CPU2		CPU3		CPU4
>> dev->struct_mutex (from #0)
>> 		mm->mmap_sem
>> 		dev->struct_mutex (from #5)
>> 				console_lock (from #4)
>> 				mm->mmap_sem
>> 						cpu_hotplug.lock (from #3)
>> 						console_lock
>> 								cma_mutex (from #2, but also from #1)
>> 								cpu_hotplug.lock
>> cma_mutex
>>
>> Which is pretty sick - and I don't think that blaming this solely on V4L2
>> nor DRM is particularly fair.  I believe the onus is on every author of
>> one of those locks involved in that chain needs to re-analyse whether
>> their locking is sane.
>>
>> For instance, what is cma_mutex protecting?  Is it protecting the CMA
>> bitmap?
>
> This lock is protecting CMA bitmap and also serializes all CMA allocations.
> It is required by memory management core to serialize all calls to
> alloc_contig_range() (otherwise page block's migrate types might get
> overwritten). I don't see any other obvious solution for serializing
> alloc_contig_range() calls.

That's unfortunate, because what you're effectively asking is for every
subsystem in the kernel to avoid a complex set of lock dependencies.  It
appears that two subsystems have now hit this, and I wouldn't be
surprised if they weren't the last.

> This will not work correctly if there will be 2 concurrent calls to  
> alloc_contig_range(), which will touch the same memory page blocks.

Can you see any other way to lessen the impact of cma_mutex on the
whole kernel?

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