(2014/12/09 19:30), Jon Medhurst (Tixy) wrote: > 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 :-)
Ah, right! stop_machine with cpu_online_mask cause that problem. Thanks, -- Masami HIRAMATSU Software Platform Research Dept. Linux Technology Research Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu...@hitachi.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/