On Fri, 30 Nov 2012 22:56:27 -0800 Michel Lespinasse <walken@xxxxxxxxxx> wrote: > expand_stack() runs with a shared mmap_sem lock. Because of this, there > could be multiple concurrent stack expansions in the same mm, which may > cause problems in the vma gap update code. > > I propose to solve this by taking the mm->page_table_lock around such vma > expansions, in order to avoid the concurrency issue. We only have to worry > about concurrent expand_stack() calls here, since we hold a shared mmap_sem > lock and all vma modificaitons other than expand_stack() are done under > an exclusive mmap_sem lock. > > I previously tried to achieve the same effect by making sure all > growable vmas in a given mm would share the same anon_vma, which we > already lock here. However this turned out to be difficult - all of the > schemes I tried for refcounting the growable anon_vma and clearing > turned out ugly. So, I'm now proposing only the minimal fix. > I think I don't understand the problem fully. Let me demonstrate: a) vma_lock_anon_vma() doesn't take a lock which is specific to "this" anon_vma. It takes anon_vma->root->mutex. That mutex is shared with vma->vm_next, yes? If so, we have no problem here? (which makes me suspect that the races lies other than where I think it lies). b) I can see why a broader lock is needed in expand_upwards(): it plays with a different vma: vma->vm_next. But expand_downwards() doesn't do that - it only alters "this" vma. So I'd have thought that vma_lock_anon_vma("this" vma) would be sufficient. What are the performance costs of this change? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>