Re: [patch] splice mmap_sem deadlock

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Linus Torvalds wrote:
> The comment is wrong.
> 
> On Mon, 1 Oct 2007, Jens Axboe wrote:
>>  
>>  /*
>> + * Do a copy-from-user while holding the mmap_semaphore for reading. If we
>> + * have to fault the user page in, we must drop the mmap_sem to avoid a
>> + * deadlock in the page fault handling (it wants to grab mmap_sem too, but for
>> + * writing). This assumes that we will very rarely hit the partial != 0 path,
>> + * or this will not be a win.
>> + */
> 
> Page faulting only grabs it for reading, and having a page fault happen is 
> not problematic in itself. Readers *do* nest.
> 
> What is problematic is:
> 
> 	thread#1			thread#2
> 
> 	get_iovec_page_array
> 	down_read()
> 	.. everything ok so far ..
> 					mmap()
> 					down_write()
> 					.. correctly blocks on the reader ..
> 					.. everything ok so far ..
> 
> 	.. pagefault ..
> 	down_read()
> 	.. fairness code now blocks on the waiting writer! ..
> 	.. oops. We're deadlocked ..
> 
> So the problem is that while readers do nest nicely, they only do so if no 
> potential writers can possibly exist (which of course never happens: an 
> rwlock with no writers is a no-op ;).

Sounds very similar to the problems we had with CPU hotplug earlier.
It's a rwlock locking anti-pattern. I know that recursive locks
have been frowned upon earlier, but I wonder if there is a case here.
Of-course recursive locks would not be *fair*.

The other solution of passing down lock ownership information is a pain.

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL

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