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Message-ID: <AANLkTim16oT13keYK_oz=7kmDmdG=ADfkGXMKp3_dEw_@mail.gmail.com> Date: Fri, 27 Aug 2010 14:28:04 -0700 From: Hugh Dickins <hughd@...gle.com> To: Christoph Lameter <cl@...ux.com> Cc: Andrea Arcangeli <aarcange@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, Rik van Riel <riel@...hat.com>, Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH] mm: fix hang on anon_vma->root->lock On Fri, Aug 27, 2010 at 1:56 PM, Christoph Lameter <cl@...ux.com> wrote: > On Fri, 27 Aug 2010, Hugh Dickins wrote: > >> Nothing ensures that the root pointer was not changed after the >> ACCESS_ONCE, that's exactly why we use ACCESS_ONCE there: once we've >> got the lock and realize that what we've locked may not be what we >> wanted (or may change from what we were wanting at any moment, the >> page no longer being mapped there - but in that case we no longer want >> it), we have to be sure to unlock the one we locked, rather than the >> one which anon_vma->root might subsequently point to. > > I do not see any check after we have taken the lock to verify that we > locked the correct object. Was there a second version of the patch? No second version of the patch, no. As I said already, it's that second page_mapped check which gives the guarantee that the anon_vma has not yet been freed, hence we've locked the correct object. > >> > Since there is no lock taken before the mapped check none of the >> > earlier reads from the anon vma structure nor the page mapped check >> > necessarily reflect a single state of the anon_vma. >> >> There's no lock (other than RCU's read "lock") taken before the >> original mapped check, and that's important, otherwise our attempt to >> lock might actually spinon or corrupt something that was long ago an >> anon_vma. But we do take the anon_vma->root->lock before the second >> mapped check which I added. If the page is still mapped at the point > > You then are using an object from the anon_vma (the pointer) without a > lock! Yes. (not counting RCU's read "lock" as a lock). > This is unstable therefore unless there are other constraints. The > anon_vma->lock must be taken before derefencing that pointer. No, SLAB_DESTROY_BY_RCU gives us just the stablity we need to take the lock. > The page may > have been unmapped and mapped again between the two checks. Unlikely but > possible. Yes, unlikely but possible. (Well, actually, is it possible? It can be unmapped on exit without any lock, but unmapping for pageout would require the page lock, would insert a swp_entry_t, and mapping again would go to do_swap_page which would again require the page lock. But never mind that, let's assume there is a way it can be unmapped and mapped again.) The thing is, page->mapping will point to the same anon_vma throughout, that only gets reset when the page is freed, and there should be nowhere else that modifies page->mapping once it's been set to anon_vma - if you know of somewhere, please point to it, we do need to examine that case. > >> of that second check, then we know that we got the right anon_vma, > > I do not see a second check (*after* taking the lock) in the patch > and the > way the lock is taken can be a problem in itself. > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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