----- On Sep 28, 2017, at 12:16 PM, Nicholas Piggin npiggin@xxxxxxxxx wrote: > On Thu, 28 Sep 2017 15:29:50 +0000 (UTC) > Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote: > >> ----- On Sep 28, 2017, at 11:01 AM, Nicholas Piggin npiggin@xxxxxxxxx wrote: >> >> > On Thu, 28 Sep 2017 13:31:36 +0000 (UTC) >> > Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote: >> > >> >> ----- On Sep 27, 2017, at 9:04 AM, Nicholas Piggin npiggin@xxxxxxxxx wrote: >> >> > > [snip] > >> >> So I don't see much point in trying to remove that registration step. >> > >> > I don't follow you. You are talking about the concept of registering >> > intention to use a different function? And the registration API is not >> > merged yet? >> >> Yes, I'm talking about requiring processes to invoke membarrier cmd >> MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED before they can successfully >> invoke membarrier cmd MEMBARRIER_CMD_PRIVATE_EXPEDITED. >> >> > Let me say I'm not completely against the idea of a registration API. But >> > don't think registration for this expedited command is necessary. >> >> Given that we have the powerpc lack-of-full-barrier-on-return-to-userspace >> case now, and we foresee x86-sysexit, sparc, and alpha also requiring >> special treatment when we introduce the MEMBARRIER_FLAG_SYNC_CORE behavior >> in the next release, it seems that we'll have a hard time handling >> architecture special cases efficiently if we don't expose the registration >> API right away. > > But SYNC_CORE is a different functionality, right? You can add the > registration API for it when that goes in. Sure, I could. However, I was hoping to re-use the same command, with a "SYNC_CORE" flag, and I would have liked to have consistent behavior for same commands used with different flags. > >> > But (aside) let's say a tif flag turns out to be a good diea for your >> > second case, why not just check the flag in the membarrier sys call and >> > do the registration the first time it uses it? >> >> We also considered that option. It's mainly about guaranteeing that >> an expedited membarrier command never blocks. If we introduce this >> "lazy auto-registration" behavior, we end up blocking the process >> at a random point in its execution so we can issue a synchronize_sched(). >> By exposing an explicit registration, we can control where this delay >> occurs, and even allow library constructors to invoke the registration >> while the process is a single threaded, therefore allowing us to completely >> skip synchronize_sched(). > > Okay I guess that could be a good reason. As I said I'm not opposed to > the concept. I suppose you could even have a registration for expedited > private even if it's a no-op on all architectures, just in case some new > ways of implementing it can be done in future. That's an approach I would be OK with too. Mandating explicit registration will give us much more flexibility. > I suppose I'm more objecting to the added complexity for powerpc, and > more code in the fastpath to make the slowpath faster. Just to make sure I understand your concern here. The "fastpath" you refer to is the TIF flag test in membarrier_sched_in() within finish_task_switch(), and the "slowpath" is switch_mm() which lacks the required full barrier now, am I correct ? Would it help if we invoke the membarrier hook from switch_mm() instead ? We'd therefore only add the TIF flag test in switch_mm(), rather than for every context switch. Thanks, Mathieu > > Thanks, > Nick -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
