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

(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 at hitachi.com