On Thu, Dec 03, 2015 at 05:21:22PM +0800, libin wrote: > > on 2015/12/2 20:36, Will Deacon wrote: > > On Sat, Nov 28, 2015 at 03:50:09PM +0800, Li Bin wrote: > >> On arm64, kstop_machine which is hugely disruptive to a running > >> system is not needed to convert nops to ftrace calls or back, > >> because that modifed code is a single 32bit instructions which > >> is impossible to cross cache (or page) boundaries, and the used str > >> instruction is single-copy atomic. > > This commit message is misleading, since the single-copy atomicity > > guarantees don't apply to the instruction-side. Instead, the architecture > > calls out a handful of safe instructions in "Concurrent modification and > > execution of instructions". > > Right, thank you for your comments. > > > Now, those safe instructions *do* include NOP, B and BL, so that should > > be sufficient for ftrace provided that we don't patch condition codes > > (and I don't think we do). > > Yes, and so far this assumption has no probem, but in order to avoid > exceeding these > safe insturctions in the future, we can use aarch64_insn_hotpatch_safe() to > verify the > instruction to determine whether needs stop_machine() to synchronize or use > aarch64_insn_patch_text directly. Right or I am missing something?
I think you're missing the case where the instruction changes under our feet after we've read it but before we've replaced it (e.g. due to module unloading). I think that's why ftrace_modify_code has the comment about lack of locking thanks to stop_machine. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
