| Date | Thu, 16 Jun 2011 08:56:45 +0530 | From | Srikar Dronamraju <> | Subject | Re: [PATCH v4 3.0-rc2-tip 7/22] 7: uprobes: mmap and fork hooks. |
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* Peter Zijlstra <peterz@infradead.org> [2011-06-15 20:11:26]:
> On Tue, 2011-06-07 at 18:29 +0530, Srikar Dronamraju wrote: > > + up_write(&mm->mmap_sem); > > + mutex_lock(&uprobes_mutex); > > + down_read(&mm->mmap_sem); > > egads, and all that without a comment explaining why you think that is > even remotely sane. > > I'm not at all convinced, it would expose the mmap() even though you > could still decide to tear it down if this function were to fail, I bet > there's some funnies there.
The problem is with lock ordering. register/unregister operations acquire uprobes_mutex (which serializes register unregister and the mmap_hook) and then holds mmap_sem for read before they insert a breakpoint.
But the mmap hook would be called with mmap_sem held for write. So acquiring uprobes_mutex can result in deadlock. Hence we release the mmap_sem, take the uprobes_mutex and then again hold the mmap_sem.
After we re-acquire the mmap_sem, we do check if the vma is valid.
Do we have better solutions?
-- Thanks and Regards Srikar
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