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

Peter Zijlstra
2007-04-06T12:26:14Z

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?