Messages in this thread | | | Date | Tue, 09 Dec 2014 19:14:55 +0900 | From | Masami Hiramatsu <> | Subject | Re: Re: [PATCH v14 7/7] ARM: kprobes: enable OPTPROBES for ARM 32 |
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(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@hitachi.com
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