Subject: Re: [PATCH v6 02/15] memory-hotplug: check whether all memory blocks are offlined or not when removing memory

From: Andrew Morton <[email-masked]>
Date: Wed, 9 Jan 2013 15:11:40 -0800

On Wed, 9 Jan 2013 17:32:26 +0800
Tang Chen <[email-masked]> wrote:

> We remove the memory like this:
> 1. lock memory hotplug
> 2. offline a memory block
> 3. unlock memory hotplug
> 4. repeat 1-3 to offline all memory blocks
> 5. lock memory hotplug
> 6. remove memory(TODO)
> 7. unlock memory hotplug
> 
> All memory blocks must be offlined before removing memory. But we don't hold
> the lock in the whole operation. So we should check whether all memory blocks
> are offlined before step6. Otherwise, kernel maybe panicked.

Well, the obvious question is: why don't we hold lock_memory_hotplug()
for all of steps 1-4?  Please send the reasons for this in a form which
I can paste into the changelog.

Actually, I wonder if doing this would fix a race in the current
remove_memory() repeat: loop.  That code does a
find_memory_block_hinted() followed by offline_memory_block(), but
afaict find_memory_block_hinted() only does a get_device().  Is the
get_device() sufficiently strong to prevent problems if another thread
concurrently offlines or otherwise alters this memory_block's state?