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

From: Oleg Nesterov
Date: Fri Apr 20 2012 - 14:38:31 EST

On 04/20, Peter Zijlstra wrote:
> On Mon, 2012-04-16 at 23:47 +0200, Oleg Nesterov wrote:
> > (And perhaps we can stop right here? I mean how often this can
> > slow down the debugger which installs int3 in the same mm?)
> >
> > Now we need to clear MMF_HAS_UPROBE somehowe, when the last
> > uprobe goes away. Lets ignore uprobe_map/unmap for simplicity.
> >
> > - We add another flag, MMF_UPROBE_RECALC, it is set by
> > remove_breakpoint().
> >
> > - We change handle_swbp(). Ignoring all details it does:
> >
> > if (find_uprobe(vaddr))
> > process_uprobe();
> > else if (test_bit(MMF_HAS_UPROBE) && test_bit(MMF_UPROBE_RECALC))
> > recalc_mmf_uprobe_flag();
> >
> > where recalc_mmf_uprobe_flag() checks all vmas and either
> > clears both flags or MMF_UPROBE_RECALC only.
> >
> > This is the really slow O(n) path, but it can only happen after
> > unregister, and only if we hit another non-uprobe breakpoint
> > in the same mm.
> >
> > Something like this. What do you think?
> I think I can live with the simple set MMF_HAS_UPROBE and leave it at
> that.

Sure, I agree.

A false positive MMF_HAS_UPROBE can only slow down the non-uprobe
int3 in the same ->mm, I think we can tolerate this.

> The better optimization seems to be to not install breakpoints
> when ->filter() excludes the task..

Ah, this is another story. And I agree this is more important.

So far I do not understand what we should do. Of course, it would
be simple to add the filtering when we install the breakpoint but
I don't think it is that simple, even if we ignore the nasty
complications with multiple consumers with different filters.

Say, a user wants to probe /sbin/init only. What if init forks?
We should remove breakpoints from child->mm somehow.

And then we also need the filtering in uprobe_mmap() at least.

But yes, I agree, it would be very nice to do this.


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