Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all_locks() | From | Peter Zijlstra <> | Date | Thu, 01 Apr 2010 18:49:52 +0200 |
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On Thu, 2010-04-01 at 18:45 +0200, Peter Zijlstra wrote: > On Thu, 2010-04-01 at 18:18 +0200, Andrea Arcangeli wrote: > > On Thu, Apr 01, 2010 at 06:12:34PM +0200, Peter Zijlstra wrote: > > > One thing we can do there is to mutex_trylock() if we get the lock, see > > > if we've got the right object, if the trylock fails we can do the > > > refcount thing and sleep. That would allow the fast-path to remain a > > > single atomic. > > > > But then how do you know which anon_vma_unlink has to decrease the > > refcount and which not? That info should need to be stored in the > > kernel stack, can't be stored in the vma. I guess it's feasible but > > passing that info around sounds more tricky than the trylock itself > > (adding params to those functions with int &refcount). > > I was thinking of something like: > > struct anon_vma *page_lock_anon_vma(struct page *page) > { > struct anon_vma *anon_vma = NULL; > unsigned long anon_mapping; > > rcu_read_lock(); > anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping); > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > goto out; > if (!page_mapped(page)) > goto out; > > anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); > if (!mutex_trylock(&anon_vma->lock)) { > if (atomic_inc_unless_zero(&anon_vma->ref)) { > rcu_read_unlock(); > mutex_lock(&anon_vma->lock); > atomic_dec(&anon_vma->ref); /* ensure the lock pins it */ > } else > anon_vma = NULL; > } > rcu_read_unlock(); > > return anon_vma; > } > > void page_unlock_anon_vma(struct anon_vma *anon_vma) > { > mutex_unlock(&anon_vma->lock); > } > > Then anybody reaching ref==0 would only need to sync against the lock > before freeing.
Ah, there is a race where the dec after lock makes it 0, we could catch that by making it -1 and free in unlock_anon_vma().
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