On Fri, Sep 10, 2021 at 10:12:01AM +0200, Rasmus Villemoes wrote: > On 08/09/2021 17.44, Luis Chamberlain wrote: > > On Tue, Sep 07, 2021 at 08:00:03PM -0700, Andrew Morton wrote: > >> From: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> > >> Any call of wait_for_initramfs() done before the unpacking has been > >> scheduled (i.e. before rootfs_initcall time) must just return > >> immediately [and let the caller find an empty file system] in order > >> not to deadlock the machine. I mistakenly thought, and my limited > >> testing confirmed, that there were no such calls, so I added a > >> pr_warn_once() in wait_for_initramfs(). It turns out that one can > >> indeed hit request_module() as well as kobject_uevent_env() during > >> those early init calls, leading to a user-visible warning in the > >> kernel log emitted consistently for certain configurations. > > > > Further proof that the semantics for init is still loose. Formalizing > > dependencies on init is something we should strive to. Eventualy with a > > DAG. The linker-tables work I had done years ago strived to get us > > there which allows us to get a simple explicit DAG through the linker. > > Unfortunately that patch set fell through because folks were > > more interested in questioning the alternative side benefits of > > linker-tables, but the use-case for helping with init is still valid. > > > > If we *do* want to resurrect this folks should let me know. > > Heh, a while back I actually had some completely unrelated thing where > I'd want to make use of the linker tables infrastructure - I remembered > reading about it on LWN, and was quite surprised when I learnt that that > work had never made it in. I don't quite remember the use case (I think > it was for some test module infrastructure). But if you do have time to > resurrect those patches, I'd certainly be interested. OK I might. > > Since the kobject_uevent_env() interest here is for /sbin/hotplug and > > that crap is deprecated, in practice the relevant calls we'd care about > > are the request_module() calls. > > Yes - the first report I got about that pr_warn_once was indeed fixed by > the reporter simply disabling CONFIG_UEVENT_HELPER > (https://lore.kernel.org/lkml/9849be80-cfe5-b33e-8224-590a4c451415@xxxxxxxxx/). Ah I see. > >> We could just remove the pr_warn_once(), but I think it's better to > >> postpone enabling the usermodehelper framework until there is at least > >> some chance of finding the executable. That is also a little more > >> efficient in that a lot of work done in umh.c will be elided. > > > > I *don't* think we were aware that such request_module() calls were > > happening before the fs was even ready and failing silently with > > -ENOENT. > > Probably not, no, otherwise somebody would have noticed. OK your commit log was not clear on this, it seemed to suggest this as a possibility or that such a case existed. That also means the impact of your change is less. > >> However, > >> it does change the error seen by those early callers from -ENOENT to > >> -EBUSY, so there is a risk of a regression if any caller care about > >> the exact error value. > > > > I'd see this as a welcomed evolution as it tells us more: we're saying > > "it's coming, try again" or whatever. > > Indeed, and I don't think it's the end of the world if somebody notices > some change due to that, because we'd learn more about where those early > request_module() calls come from. But since it would seem none have been reported yet, it is an even better situation. > > A debug option to allow us to get a full warning trace in the -EBUSY > > case on early init would be nice to have. > > As noted above, that's difficult. We'd need a way to know which other > task is waiting for us, then print the trace of that guy. > > I don't think anybody is gonna hear this tree falling, so let's not try > to solve a problem before we know there is one. That's fair. But let's also recall neither of us expected the above situation either. But I agree the possible collateral at this point seems to be small, if any. Luis
