linux-kernel - Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu

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Date:	Mon, 16 Apr 2012 23:47:07 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anton Arapov <anton@...hat.com>
Subject: Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id

On 04/16, Peter Zijlstra wrote:
>
> On Mon, 2012-04-16 at 01:44 +0200, Oleg Nesterov wrote:
>
> > And. I have another reason for down_write() in register/unregister.
> > I am still not sure this is possible (I had no time to try to
> > implement), but it seems to me we can kill the uprobe counter in
> > mm_struct.
>
> You mean by making register/unregister down_write, you're exclusive with
> munmap()

.. and with register/unregister.

Why do we need mm->uprobes_state.count? It is writeonly, except we
check it in the DIE_INT3 notifier before anything else to avoid the
unnecessary uprobes overhead.

Suppose we kill it, and add the new MMF_HAS_UPROBE flag instead.
install_breakpoint() sets it unconditionally,
uprobe_pre_sstep_notifier() checks it.

(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?

Oleg.

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