Vlad Yasevich <vyasevich@xxxxxxxxx> writes: > On 12/04/2015 07:55 AM, Marcelo Ricardo Leitner wrote: >> On Fri, Dec 04, 2015 at 11:40:02AM +0100, Dmitry Vyukov wrote: >>> On Thu, Dec 3, 2015 at 9:51 PM, Joe Perches <joe@xxxxxxxxxxx> wrote: >>>> (adding lkml as this is likely better discussed there) >>>> >>>> On Thu, 2015-12-03 at 15:42 -0500, Jason Baron wrote: >>>>> On 12/03/2015 03:24 PM, Joe Perches wrote: >>>>>> On Thu, 2015-12-03 at 15:10 -0500, Jason Baron wrote: >>>>>>> On 12/03/2015 03:03 PM, Joe Perches wrote: >>>>>>>> On Thu, 2015-12-03 at 14:32 -0500, Jason Baron wrote: >>>>>>>>> On 12/03/2015 01:52 PM, Aaron Conole wrote: >>>>>>>>>> I think that as a minimum, the following patch should be evaluted, >>>>>>>>>> but am unsure to whom I should submit it (after I test): >>>>>>>> [] >>>>>>>>> Agreed - the intention here is certainly to have no side effects. It >>>>>>>>> looks like 'no_printk()' is used in quite a few other places that would >>>>>>>>> benefit from this change. So we probably want a generic >>>>>>>>> 'really_no_printk()' macro. >>>>>>>> >>>>>>>> https://lkml.org/lkml/2012/6/17/231 >>>>>>> >>>>>>> I don't see this in the tree. >>>>>> >>>>>> It never got applied. >>>>>> >>>>>>> Also maybe we should just convert >>>>>>> no_printk() to do what your 'eliminated_printk()'. >>>>>> >>>>>> Some of them at least. >>>>>> >>>>>>> So we can convert all users with this change? >>>>>> >>>>>> I don't think so, I think there are some >>>>>> function evaluation/side effects that are >>>>>> required. I believe some do hardware I/O. >>>>>> >>>>>> It'd be good to at least isolate them. >>>>>> >>>>>> I'm not sure how to find them via some >>>>>> automated tool/mechanism though. >>>>>> >>>>>> I asked Julia Lawall about it once in this >>>>>> thread: https://lkml.org/lkml/2014/12/3/696 >>>>>> >>>>> >>>>> Seems rather fragile to have side effects that we rely >>>>> upon hidden in a printk(). >>>> >>>> Yup. >>>> >>>>> Just convert them and see what breaks :) >>>> >>>> I appreciate your optimism. It's very 1995. >>>> Try it and see what happens. >>> >>> >>> Whatever is the resolution for pr_debug, we still need to fix this >>> particular use-after-free. It affects stability of debug builds, gives >>> invalid debug output, prevents us from finding more bugs in SCTP. And >>> maybe somebody uses CONFIG_DYNAMIC_DEBUG in production. >> >> Agreed. I'm already working on a fix for this particular use-after-free. >> >> Another interesting thing about this is that sctp_do_sm() is called for >> nearly every movement that happens on a sctp socket. Said that, that >> always-running IDR search hidden on that debug statement do have some >> nasty performance impact, specially because it's serialized on a >> spinlock. > > YUCK! I didn't really pay much attention to those debug macros before, but > debug_post_sfx() is truly awful. > > This wasn't such a bad thing where these macros depended on CONFIG_SCTP_DEBUG, > but now that they are always built, we need fix them. I've proposed a patch to linux-kernel to fix them, but I don't think it's really as bad as folks imagine. Ubuntu, RHEL, and Fedora all use DYNAMIC_DEBUG configuration option, which means that the code is getting emitted anyway (correctly, I'll add) and is shunted out by a dynamic debug flag. So for the average user, it's not even really a blip. That does mean there's a cool side-effect of the entire print-macro setup which implies we execute less code when running with DYNAMIC_DEBUG=y in the "normal" case. "Turn on the dynamic debugging config and watch everything get better" isn't the worst mantra, is it? :) > -vlad > > > >> This wouldn't be happening if it was fully ellided and would >> be ok if that pr_debug() was really being printed, but not as it is. >> Kudos to this report that I could notice this. I'm trying to fix this on >> SCTP-side as well. >> >> Marcelo >> -- To unsubscribe from this list: send the line "unsubscribe linux-sctp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
