Messages in this thread | | | Date | Thu, 09 Aug 2018 08:18:50 -0400 | Subject | Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage | From | joel@joelfern ... |
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On August 8, 2018 6:47:16 PM EDT, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: >On Wed, Aug 08, 2018 at 03:15:31PM -0700, Joel Fernandes wrote: >> On Wed, Aug 8, 2018 at 1:18 PM, Paul E. McKenney >> <paulmck@linux.vnet.ibm.com> wrote: >> [...] >> >> >> >> It does start to seem like a show stopper :-( >> >> >> > >> >> >> > I suppose that an srcu_read_lock_nmi() and >srcu_read_unlock_nmi() could >> >> >> > be added, which would do atomic ops on >sp->sda->srcu_lock_count. Not sure >> >> >> > whether this would be fast enough to be useful, but easy to >provide: >> >> >> > >> >> >> > int __srcu_read_lock_nmi(struct srcu_struct *sp) /* >UNTESTED. */ >> >> >> > { >> >> >> > int idx; >> >> >> > >> >> >> > idx = READ_ONCE(sp->srcu_idx) & 0x1; >> >> >> > atomic_inc(&sp->sda->srcu_lock_count[idx]); >> >> >> > smp_mb__after_atomic(); /* B */ /* Avoid leaking >critical section. */ >> >> >> > return idx; >> >> >> > } >> >> >> > >> >> >> > void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx) >> >> >> > { >> >> >> > smp_mb__before_atomic(); /* C */ /* Avoid leaking >critical section. */ >> >> >> > atomic_inc(&sp->sda->srcu_unlock_count[idx]); >> >> >> > } >> >> >> > >> >> >> > With appropriate adjustments to also allow Tiny RCU to also >work. >> >> >> > >> >> >> > Note that you have to use _nmi() everywhere, not just in NMI >handlers. >> >> >> > In fact, the NMI handlers are the one place you -don't- need >to use >> >> >> > _nmi(), strangely enough. >> >> >> > >> >> >> > Might be worth a try -- smp_mb__{before,after}_atomic() is a >no-op on >> >> >> > some architectures, for example. >> >> >> >> >> >> Continuing Steve's question on regular interrupts, do we need >to use >> >> >> this atomic_inc API for regular interrupts as well? So I guess >> >> > >> >> > If NMIs use one srcu_struct and non-NMI uses another, the >current >> >> > srcu_read_lock() and srcu_read_unlock() will work just fine. If >any given >> >> > srcu_struct needs both NMI and non-NMI readers, then we really >do need >> >> > __srcu_read_lock_nmi() and __srcu_read_unlock_nmi() for that >srcu_struct. >> >> >> >> Yes, I believe as long as in_nmi() works reliably, we can use the >> >> right srcu_struct (NMI vs non-NMI) and it would be fine. >> >> >> >> Going through this thread, it sounds though that this_cpu_inc may >not >> >> be reliable on all architectures even for non-NMI interrupts and >> >> local_inc may be the way to go. >> > >> > My understanding is that this_cpu_inc() is defined to handle >interrupts, >> > so any architecture on which it is unreliable needs to fix its bug. > ;-) >> >> Yes that's my understanding as well. >> >> Then may be I'm missing something about yours/Steve's conversations >in >> the morning, about why we need bother with the local_inc then. So the >> current SRCU code with the separate NMI handle should work fine (for >> future merge windows) as long as we're using a separate srcu_struct >> for NMI. :-) > >I do believe that to be true. But only as long as that separate >srcu_struct is -only- used for NMI. > >If this is what you have been pushing for all along, please accept my >apologies for my being slow! >
That's ok, sorry I initially didn't describe it well which may have caused confusion, but yes that's what I was pushing for.
>That said, your approach does require you to have a perfect way to >distinguish between NMI and not-NMI. If the distinguishing is even >in the slightest imperfect, then some sort of NMI-safe SRCU reader >approach is of course required. >
Thanks Paul, agreed with everything and we are on the same page.
- Joel
> Thanx, Paul > >> >> For next merge window (not this one), lets do that then? Paul, if >you >> >> could provide me an SRCU API that uses local_inc, then I believe >that >> >> coupled with this patch should be all that's needed: >> >> https://lore.kernel.org/patchwork/patch/972657/ >> >> >> >> Steve did express concern though if in_nmi() works reliably (i.e. >> >> tracepoint doesn't fire from "thunk" code before in_nmi() is >> >> available). Any thoughts on that Steve? >> > >> > Agreed, not the upcoming merge window. But we do need to work out >> > exactly what is the right way to do this. >> >> Agreed, thanks! >> >> - Joel >>
-- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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