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 18:45:00 +0200 |
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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.
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