Re: [PATCH v5 3.1.0-rc4-tip 18/26] uprobes: slot allocation.

* Peter Zijlstra [off-list ref] [2011-09-27 14:18:52]:

On Tue, 2011-09-20 at 17:33 +0530, Srikar Dronamraju wrote:

quoted

+static struct uprobes_xol_area *xol_alloc_area(void)
+{
+       struct uprobes_xol_area *area = NULL;
+
+       area = kzalloc(sizeof(*area), GFP_KERNEL);
+       if (unlikely(!area))
+               return NULL;
+
+       area->bitmap = kzalloc(BITS_TO_LONGS(UINSNS_PER_PAGE) * sizeof(long),
+                                                               GFP_KERNEL);
+
+       if (!area->bitmap)
+               goto fail;
+
+       init_waitqueue_head(&area->wq);
+       spin_lock_init(&area->slot_lock);
+       if (!xol_add_vma(area) && !current->mm->uprobes_xol_area) {

So what happens if xol_add_vma() succeeds, but we find
->uprobes_xol_area set?

quoted

+               task_lock(current);
+               if (!current->mm->uprobes_xol_area) {

Having to re-test it under this lock seems to suggest it could.

quoted

+                       current->mm->uprobes_xol_area = area;
+                       task_unlock(current);
+                       return area;

This function would be so much easier to read if the success case (this
here I presume) would not be nested 2 deep.

quoted

+               }
+               task_unlock(current);
+       }

at which point you could end up with two extra vmas? Because there's no
freeing of the result of xol_add_vma().

Agree, we need to unmap the vma in that case.

quoted

+fail:
+       kfree(area->bitmap);
+       kfree(area);
+       return current->mm->uprobes_xol_area;
+} 

-- 
Thanks and Regards
Srikar

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