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Date: Mon, 16 Apr 2012 01:44:01 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, Linux-mm <linux-mm@kvack.org>,
        Andi Kleen <andi@firstfloor.org>,
        Christoph Hellwig <hch@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Thomas Gleixner <tglx@linutronix.de>, Anton Arapov <anton@redhat.com>
Subject: Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id
Message-ID: <20120415234401.GA32662@redhat.com>
References: <20120405222024.GA19154@redhat.com> <1334409396.2528.100.camel@twins> <20120414205200.GA9083@redhat.com> <1334487062.2528.113.camel@twins> <20120415195351.GA22095@redhat.com> <1334526513.28150.23.camel@twins>
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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.