On Tue, Jul 04, 2017 at 10:12:13PM +0200, Rafael J. Wysocki wrote: > On Tue, Jul 4, 2017 at 10:05 PM, Krzysztof Kozlowski <krzk@xxxxxxxxxx> wrote: >> >> > Thanks for report! > >> >> > > >> >> > Damn it, although I couldn't find this in the code, but I was fearing > >> >> > that this ends up in atomic section. That would kind of explain why > >> >> > mutex was not there [1]. > >> >> > > >> >> > Anyway, the buggy code was there already. Instead of "sleeping in atomic > >> >> > section" there was no locking at all... In this context this was > >> >> > probably safe because it was executed *after* disabling non-boot CPUs > >> >> > but then the function cannot be called in other contexts. > >> >> > > >> >> > I am not sure I will be capable of developing the proper fix as I do not > >> >> > have the hardware and I do not know all stuff happening in sh suspend. > >> >> > Probably reverting this and living with non-locked path would be the > >> >> > safest choice. > >> >> > > >> >> > [1] https://patchwork.kernel.org/patch/9778903/ > >> >> > >> >> AFAIU, all syscore stuff runs in atomic context. > >> > > >> > Indeed... The confusing part is that this code is syscore only from > >> > the name, it is not hooked in to syscore_ops. Although going by call > >> > chain (through sh clocksource drivers) we end up in > >> > timekeeping_suspend() which is a syscore op. > >> > > >> > I wonder whether it would be useful - after reverting my commit - to add > >> > an assert (which is a stronger API requirement than only documentation "may > >> > only be called during the system core (syscore) suspend") like: > >> > WARN_ON(num_online_cpus() > 1)); > >> > as without mutexes this should not be executed with more than one online > >> > CPU. > >> > >> Or maybe WARN_ON_ONCE(!in_atomic())? > > > > You could be in atomic section on this CPU and still have other CPUs > > online playing with gpd_list (without any protection from locking). > > This function is safe only on non-SMP case. > > Well, not quite. > > It is safe if you can guarantee that no other CPUs will touch the data > structure in question concurrently, which pretty much is the case for > timekeeping_suspend() even though it may be invoked without taking the > other CPUs offline (from the suspend-to-idle core path). Right, that would work fine for that case. However I was rather thinking that we have an in-kernel API (exported) so someone might by mistake try to use it in different contexts. For example in some atomic section but on a platform which offlines CPUs later. Thus it would be called in some imaginary suspend path but with CPUs still being online. Partially it is already mentioned in documentation although I am not sure that on every possible architecture syscore ops are called after disabling non-boot CPUs... And anyway for in-kernel API having a explicit assert is a stronger way of documenting requirement than a comment. Best regards, Krzysztof
