Messages in this thread | | | Date | Fri, 18 Jan 2019 11:02:29 +0100 | From | Peter Zijlstra <> | Subject | Re: Question about qspinlock nest |
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On Mon, Jan 14, 2019 at 01:54:49PM +0000, James Morse wrote: > On 14/01/2019 13:16, Peter Zijlstra wrote:
> > What avoids the trivial self-recursion: > > > > spin_lock(&) > > <NMI> > > spin_lock(&x) > > ... wait forever more ... > > </NMI> > > spin_unlock(&x) > > > > ? > > If its trying to take the same lock, I agree its deadlocked. > If the sequence above started with <NMI>, I agree its deadlocked. > > APEI/GHES is doing neither of these things. It take a lock that is only ever > taken in_nmi(). nmi_enter()s BUG_ON(in_nmi()) means these never become re-entrant.
Urgh.. yes. I abhor that spinlock usage, but you're correct. If they're only ever used in the NMI then it ought to work.
/me digs around... Bugger we have more like that :/
> What is the lock doing? Protecting the 'NMI' fixmap slot in the unlikely case > that two CPUs end up in here at the same time. > > (I though x86's NMI masked NMI until the next iret?)
Correct; x86 has his 'feature' where IRET will unmask the NMI, so we have something quite terrible to deal with that, don't ask and I shall not have to explain :-)
> This is murkier on arm64 as we have multiple things that behave like this, but > there is an order to them, and none of them can interrupt themselves.
Well, x86 too has multiple non-maskable vectors, and afaik only the actual NMI vector is covered in tricky. But our MCE vector is non-maskable too (and I have vague memories of there being more).
Boris, Rostedt, WTH happens if our MCE code goes and hits a #BP ? (not unlikely with this proliferation of self-modifying code)
Anyway, the idea is that they can indeed not interrupt themselves, but I would not be surprised if the whole MCE thing is riddled with fail (on x86).
> e.g. We can't take an SError during the SError handler. > > But we can take this SError/NMI on another CPU while the first one is still > running the handler. > > These multiple NMIlike notifications mean having multiple locks/fixmap-slots, > one per notification. This is where the qspinlock node limit comes in, as we > could have more than 4 contexts.
Right; so Waiman was going to do a patch that reverts to test-and-set or something along those lines once we hit the queue limit, which seems like a good way out. Actually hitting that nesting level should be exceedingly rare.
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