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 -- 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/