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

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Message-ID: <20120415234401.GA32662@redhat.com>
Date:	Mon, 16 Apr 2012 01:44:01 +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

I'll write another email tomorrow, just one note...

On 04/15, Peter Zijlstra wrote:
>
> OK, still not seeing how your proposal could work.. consider the below
> patch comment, I'm not seeing how is_swbp_at_addr_fast() deals with an
> in-progress INT3 while we remove the probe.
>
> By ensuring the non-race with reg/unreg it will either find the uprobe
> (no problem)

Yes,

> or not find it and not see a breakpoint instruction either,
> even though the pending breakpoint was generated by a uprobe (which is
> now gone),

Yes,

> causing a false positive SIGTRAP.

No. Please note that if is_swbp_at_addr_fast() sets is_swbp == 0 we
restart this insn.

(note that we also restart if get_user_pages() fails, this is hopefully
 is more correct too but minor).

> Or am I still not getting it?

My experience shows this is very unlikely. I am starting to think
I missed something, will re-check.


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.

Oleg.

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