On Tue, Feb 01, 2022 at 08:13:14AM -1000, Tejun Heo wrote: > Hello, > > On Fri, Jan 28, 2022 at 09:39:12AM +0200, Linus Torvalds wrote: > > However, we've done this for *so* long that I wonder if there might be > > situations that have ended up depending on the lack of synchronization > > for pure performance reasons. > > > > If *this* module loading process started the async work, then we'd > > wait for it, but what if there's other async work that was started by > > others? This revert would now make us wait for that async work too, > > and that might be a big deal slowing things down at boot time. > > > > Looking at it, this is all under the 'module_mutex', so I guess we are > > already single-threaded at least wrt loading other modules, so the > > amount of unrelated async work going on is presumably fairly low and > > that isn't an issue. > > Looks like we're multi-threaded while running the mod inits which launch the > async jobs and single-threaded while waiting for them to finish. Greg should > know a lot better than me but according to my hazy memory and cursory code > reading udev is multi-processed when loading modules, which makes it a lot > less likely that this will impact boot time in most cases. I think userspace is multi-processed here, which should help with the reading of the modules from disk at boot while others are actually being loaded due to the kernel lock. > > Anyway, I think this patch is the right thing to do, but just the fact > > that we've avoided that async wait for so long makes me a bit nervous > > about fallout from the revert. > > > > Comments? Maybe this is a "just apply it, see if somebody screams" situation? > > So, yeah, I think the risk is pretty low and even in the unlikely case that > someone is affected, the workaround is pretty straight-forward - not waiting > for the module loading to finish if appropriate. I agree with Linus, let's see if anyone notices :) thanks, greg k-h
