Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes: > Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes: >>> The nature of this bug is also described here (with different consequences): >>> https://lore.kernel.org/lkml/20200211141554.24181-1-qais.yousef@xxxxxxx/ >> >> Yeah, pesky deadlocks.. someone was going to try again. > > I dug up the synchronous patch > > https://lore.kernel.org/lkml/1579878449-10164-1-git-send-email-prsood@xxxxxxxxxxxxxx/ > > but surprisingly wasn't able to reproduce the lockdep splat from > > https://lore.kernel.org/lkml/F0388D99-84D7-453B-9B6B-EEFF0E7BE4CC@xxxxxx/ > > even though I could hit it a few weeks ago. oh okay, you need to mount a legacy cpuset hierarchy. So as the above splat shows, making cpuset_hotplug_workfn() synchronous means cpu_hotplug_lock (and "cpuhp_state-down") can be acquired before cgroup_mutex. But there are at least four cgroup paths that take the locks in the opposite order. They're all the same, they take cgroup_mutex and then cpu_hotplug_lock later on to modify one or more static keys. cpu_hotplug_lock should probably be ahead of cgroup_mutex because the latter is taken in a hotplug callback, and we should keep the static branches in cgroup, so the only way out I can think of is moving cpu_hotplug_lock to just before cgroup_mutex is taken and switching to _cpuslocked flavors of the static key calls. lockdep quiets down with that change everywhere, but it puts another big lock around a lot of cgroup paths. Seems less heavyhanded to go with this RFC. What do you all think? Absent further discussion, Alexey, do you plan to post another version?
