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    SubjectRe: Re: [PATCH v14 7/7] ARM: kprobes: enable OPTPROBES for ARM 32
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    On Tue, 2014-12-09 at 19:14 +0900, Masami Hiramatsu wrote:
    > (2014/12/08 20:50), Jon Medhurst (Tixy) wrote:> arch_optimize_kprobes is calling __arch_optimize_kprobes, which is
    > > iterating over a list of probes and removing each one in turn, if this
    > > is happening on multiple cpu's simultaneously, it's not clear to me that
    > > such an operation is safe. list_del_init calls __list_del which does
    > >
    > > next->prev = prev;
    > > prev->next = next;
    > >
    > > so what happens if another cpu is at the same time updating any of those
    > > list entries? Without even fully analysing the code I can see that with
    > > the fact that the list handling helpers have no memory barriers, that
    > > the above two lines could be seen to execute in the reverse order, e.g.
    > >
    > > prev->next = next;
    > > next->prev = prev;
    > >
    > > so another CPU could find and delete next before this one has finished
    > > doing so. Would the list end up in a consistent state where no loops
    > > develop and no probes are missed? I don't know the answer and a full
    > > analysis would be complicated, but my gut feeling is that if a cpu can
    > > observe the links in the list in an inconsistent state then only bad
    > > things can result.
    >
    > Just a comment, arch_optimize_kprobes() are only called under
    > kprobe_mutex held. No concurrent update happens :)

    Except in the case of the code I was commenting on which was using
    stop_machine to make all cpu's simultaneously do the work of
    arch_optimize_kprobes :-)

    --
    Tixy



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