On Fri, 2007-04-06 at 14:02 +0100, 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: > > > > - do a page table walk to find the pte; > > ("walk" meaning descent down the levels, I presume, rather than across)
indeed. > I've not had time to digest your proposal, and I'm about to go out: > let me sound a warning that springs to mind, maybe it's entirely > inapproriate, but better said than kept silent. > > It looks as if you're supposing that mmap_sem is needed to find_vma, > but not for going down the pagetables. It's not a simple as that: > you need to be careful that a concurrent munmap from another thread > isn't freeing pagetables from under you. > > Holding (down_read) of mmap_sem is one way to protect against that. > try_to_unmap doesn't have that luxury: in its case, it's made safe > by the way free_pgtables does anon_vma_unlink and unlink_file_vma > before freeing any pagetables, so try_to_unmap etc. won't get there; > but you can't do that. Ah, drad. I had hoped that the pte_lock would solve that race, but that doesn't cover the upper levels of the tree. Back to the drawing board.. - 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/