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SubjectRe: [RFC] [PATCH 2.6.37-rc5-tip 8/20] 8: uprobes: mmap and fork hooks.
* Peter Zijlstra <peterz@infradead.org> [2011-01-26 11:20:39]:

> On Wed, 2011-01-26 at 14:33 +0530, Srikar Dronamraju wrote:
> >
> >
> > I actually dont like to release the write_lock and then reacquire it.
> > write_opcode, which is called thro install_uprobe, i.e to insert the
> > actual breakpoint instruction takes a read lock on the mmap_sem.
> > Hence uprobe_mmap gets called in context with write lock on mmap_sem
> > held, I had to release it before calling install_uprobe.
>
> Ah, right, so that's going to give you a head-ache ;-)
>
> The moment you release this mmap_sem, the map you're going to install
> the probe point in can go away.
>
> The only way to make this work seems to start by holding the mmap_sem
> for writing and make a breakpoint install function that assumes its
> taken and doesn't try to acquire it again.
>


Yes, this can be done.
I would have to do something like this in register_uprobe().

list_for_each_entry_safe(mm, tmpmm, &tmp_list, uprobes_list) {
down_read(&mm->map_sem);
if (!install_uprobe(mm, uprobe))
ret = 0;
up_read(&mm->map_sem);
list_del(&mm->uprobes_list);
mmput(mm);
}

Agree that this is much better than what we have now.


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