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

Nick Piggin
2007-04-06T13:15:48Z

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...

>> - 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?

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).

--
SUSE Labs, Novell Inc.