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. > 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. Thanks. -- tejun
