Excerpts from Andy Lutomirski's message of July 14, 2020 1:48 am: > On Mon, Jul 13, 2020 at 7:13 AM Mathieu Desnoyers > <mathieu.desnoyers@xxxxxxxxxxxx> wrote: >> >> ----- On Jul 13, 2020, at 9:47 AM, Nicholas Piggin npiggin@xxxxxxxxx wrote: >> >> > Excerpts from Nicholas Piggin's message of July 13, 2020 2:45 pm: >> >> Excerpts from Andy Lutomirski's message of July 11, 2020 3:04 am: >> >>> Also, as it stands, I can easily see in_irq() ceasing to promise to >> >>> serialize. There are older kernels for which it does not promise to >> >>> serialize. And I have plans to make it stop serializing in the >> >>> nearish future. >> >> >> >> You mean x86's return from interrupt? Sounds fun... you'll konw where to >> >> update the membarrier sync code, at least :) >> > >> > Oh, I should actually say Mathieu recently clarified a return from >> > interrupt doesn't fundamentally need to serialize in order to support >> > membarrier sync core. >> >> Clarification to your statement: >> >> Return from interrupt to kernel code does not need to be context serializing >> as long as kernel serializes before returning to user-space. >> >> However, return from interrupt to user-space needs to be context serializing. >> > > Indeed, and I figured this out on the first read through because I'm > quite familiar with the x86 entry code. But Nick somehow missed this, > and Nick is the one who wrote the patch. > > Nick, I think this helps prove my point. The code you're submitting > may well be correct, but it's unmaintainable. It's not. The patch I wrote for x86 is a no-op, it just moves existing x86 hook and code that's already there to a different name. Actually it's not quite a no-op, it't changes it to use hooks that are actually called in the right places. Because previously it was unmaintainable from point of view of generic mm -- it was not clear at all that the old one should have been called in other places where the mm goes non-lazy. Now with the exit_lazy_tlb hook, it can quite easily be spotted where it is missing. And x86 keeps their membarrier code in x86, and uses nice well defined lazy tlb mm hooks. > At the very least, this > needs a comment explaining, from the perspective of x86, *exactly* > what exit_lazy_tlb() is promising, why it's promising it, how it > achieves that promise, and what code cares about it. Or we could do > something with TIF flags and make this all less magical, although that > will probably end up very slightly slower. It's all documented there in existing comments plus the asm-generic exit_lazy_tlb specification added AFAIKS. Is the membarrier comment in finish_task_switch plus these ones not enough? Thanks, Nick
