Shared futexes (was [PATCH] FUTEX : new PRIVATE futexes)

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

Hi,

some thoughts on shared futexes;

Could we get rid of the mmap_sem on the shared futexes in the following
manner:

 - do a page table walk to find the pte;
 - get a page using pfn_to_page (skipping VM_PFNMAP)
 - get the futex key from page->mapping->host and page->index
   and offset from addr % PAGE_SIZE.

or given a key:

 - lookup the page from key.shared.inode->i_mapping by key.shared.pgoff
   possibly loading the page using mapping->a_ops->readpage().

then:

 - perform the futex operation on a kmap of the page


This should all work except for VM_PFNMAP.

Since the address is passed from userspace we cannot trust it to not
point into a VM_PFNMAP area.

However, with the RCU VMA lookup patches I'm working on we could do that
check without holding locks and without exclusive cachelines; the
question is, is that good enough?

Or is there an alternative way of determining a pfnmap given a
pfn/struct page?