Re: Re: [PATCH v14 7/7] ARM: kprobes: enable OPTPROBES for ARM 32

[Masami Hiramatsu]

(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 :)

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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