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SubjectRe: [PATCH] futex: Reduce the scope of lock_page, aka lockless futex_get_key()
On Tue, 05 Jan 2016, Bueso wrote:

>On Tue, 05 Jan 2016, Peter Zijlstra wrote:
>
>>On Tue, Jan 05, 2016 at 12:23:55PM -0800, Davidlohr Bueso wrote:
>>>+ if (unlikely(!mapping)) {
>>>+ int shmem_swizzled;
>>>+
>>>+ /*
>>>+ * Page lock is required to identify which special case above
>>>+ * applies. If this is really a shmem page then the page lock
>>>+ * will prevent unexpected transitions.
>>>+ */
>>>+ lock_page(page);
>>>+ shmem_swizzled = PageSwapCache(page);
>>> unlock_page(page);
>>> put_page(page);
>>>+ WARN_ON_ONCE(mapping);
>>
>>We've not re-loaded mapping, so how could this possibly be?
>
>Yep, this wants to be page->mapping.

Actually under compound_head() afaict, along with the rest of the page->mapping
loads.


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