Messages in this thread | | | Date | Thu, 4 Sep 2003 18:59:39 +0100 | From | Jamie Lokier <> | Subject | Re: [PATCH 2] Little fixes to previous futex patch |
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Hugh Dickins wrote: > In sys_remap_file_pages, you set the VM_NONLINEAR flag, then clear > it if this particular population matches the vma. No, you cannot > clear that flag once set, without checking every page and pte_file > already set within the vma. Check if population matches vma first, > and if it doesn't match just set the VM_NONLINEAR flag in that case. > (Andrew already mentioned locking: I'd have said page_table_lock, > but his mmap_sem is also appropriate: it's an odd case.)
I don't see why you can't clear the flag: the call to ->populate will change every page and pte_file to correspond with the linear page offsets, which is all that !VM_NONLINEAR indicates.
However, it _is_ wrong to clear VM_NONLINEAR before the call to ->populate() has finished, with Andrew's patch which uses downgrade_write(). Instead, the clear must come after ->populate() has finished.
> I think rip out the FIXADDR_USER_START bit, it's rather over-the-top, > ugly: and that area is readonly, so not a useful place for a futex.
Agreed. I put it because the old futex has it as a side effect of get_user_pages(). It can go.
> The units of keys[1]: bytes if private but pages if shared. > That's okay for now I think, but if a hashing expert comes along > later s/he'll probably want to change it. The current hash does > add key1 to offset, which is okay: if it xor'ed you'd lose the > the offset bits in the private case.
Feel free to think up a better hash that isn't slow. Two iterations of hash_long() would be a good hash, but slower.
> Those keys[1] pages: in units of PAGE_SIZE in the linear case, > of PAGE_CACHE_SIZE in the nonlinear case. Oh well, this is far > from the only place with such an inconsistency, let's worry > about that when never comes.
Ew.
> The err at the end of __get_page_keys would be 1 from successful > get_user_pages, treated as error by the callers: need to make it 0.
Well spotted.
> futex_wait: I didn't get around to it in my version, so haven't > thought through the issues, but I'm a bit worried that you get > curval for -EWOULDBLOCK check without holding the futex_lock. > That looks suspicious to me, but I'm going to be lazy and not > try to think about it, because Rusty is sure to understand the > races there. If that code is insufficient as you have it, may > need __pin_page reinstated for just that case (hmm, was that > get_user right before? I'd expect it to kmap_atomic pinned page.)
The important things are that the futex is queued prior to checking curval, the requested page won't change (it's protected by mmap_sem), and any parallel waker changes the word prior to waking us.
You made me notice a rather subtle memory ordering condition, though.
We must issue the read after queuing the futex. There needs to be a smp_rmb() after queuing and before the read, because the spin_unlock() barrier only constrains earlier reads, not later ones.
Thanks for all your great insights, -- Jamie - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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