Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id

From: Peter Zijlstra
Date: Sat Apr 14 2012 - 09:17:02 EST


On Fri, 2012-04-06 at 00:20 +0200, Oleg Nesterov wrote:
> Hello.
>
> Not for inclusion yet, only for the early review.
>
> I didn't even try to test these changes, and I am not expert
> in this area. And even _if_ this code is correct, I need to
> re-split these changes anyway, update the changelogs, etc.
>
> Questions:
>
> - does it make sense?

Maybe, upside is reclaiming that int from task_struct, downside is that
down_write :/ It would be very good not to have to do that. Nor do I
really see how that works.

> - can it work or I missed something "in general" ?

So we insert in the rb-tree before we take mmap_sem, this means we can
hit a non-uprobe int3 and still find a uprobe there, no?

> Why:
>
> - It would be nice to remove a member from task_struct.
>
> - Afaics, the usage of uprobes_srcu does not look right,
> at least in theory, see 6/6.
>
> The comment above delete_uprobe() says:
>
> The current unregistering thread waits till all
> other threads have hit a breakpoint, to acquire
> the uprobes_treelock before the uprobe is removed
> from the rbtree.
>
> but synchronize_srcu() can only help if a thread which
> have hit the breakpoint has already called srcu_read_lock().
> It can't synchronize with read_lock "in future", and there
> is a small window.
>
> We could probably add another synchronize_sched() before
> synchronize_srcu(), but this doesn't look very nice and

Right, I think that all was written with the assumption that sync_srcu
implied a sync_rcu, which of course we've recently wrecked.


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