On Fri, 2007-04-06 at 23:15 +1000, Nick Piggin wrote: > Hugh Dickins wrote: > > On Fri, 6 Apr 2007, Peter Zijlstra wrote: > > > >>some thoughts on shared futexes; > >> > >>Could we get rid of the mmap_sem on the shared futexes in the following > >>manner: > > I'd imagine shared futexes would be much less common than private for > threaded programs... I'd say we should reevaluate things once we have > private futexes, and malloc/free stop hammering mmap_sem so hard...
Indeed, private futexes are by far the most common. > >> - 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(). > > For shared futexes, wouldn't i_mapping be worse, because you'd be > ping-ponging the tree_lock between processes, rather than have each > use their own mmap_sem? Your lockless pagecache work would solve most of that, no? > That also only helps for the wakeup case too, doesn't it? You have > to use the vmas to find out which inode to use to do the wait, I think? > (unless you introduce a new shared futex API). one could do something like this: struct address_space *mapping = page_mapping(page); if (!mapping || mapping == &swapper_space) do_private_futex(); else do_shared_futex(mapping->host, page->index, addr % PAGE_SIZE); But alas, it seems I overlooked that the mmap_sem also protects the page tables as pointed out by Hugh, so this is all in fain it seems. A well. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
