Messages in this thread | | | Date | Mon, 20 Apr 2015 08:53:58 -0700 | Subject | Re: qemu:arm test failure due to commit 8053871d0f7f (smp: Fix smp_call_function_single_async() locking) | From | Linus Torvalds <> |
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On Sun, Apr 19, 2015 at 10:39 PM, Ingo Molnar <mingo@kernel.org> wrote: > >> >> So I _could_ imagine that somebody would want to do optimistic "prod >> other cpu" calls that in all normal cases are for existing cpus, but >> could be racy in theory. > > Yes, and I don't disagree with such optimizations in principle (it > allows less references to be taken in the fast path), but is it really > safe? > > If a CPU is going down and we potentially race against that, and send > off an IPI, the IPI might be 'in flight' for an indeterminate amount > of time, especially on wildly non-deterministic hardware like virtual > platforms.
Well, it should be easy enough to handle that race in the cpu offlining: after the cpu is marked "not present", just call flush_smp_call_function_queue(), In fact, I thought we did exactly that - it's the reason for the "warn_cpu_offline" argument, isn't it)?
So I don't think there should be any real race. Sure, the HW IPI itself might be in flight, but from a sw perspective isn't all done.
No, I was talking about something even more optimistic - the CPU number we optimisitcally loaded and sent an IPI to might be completely bogus just because we loaded it using some unlocked sequence, and maybe the memory got re-assigned. So it might not even be a CPU number that is "stale", it could be entirely invalid.
And no, I don't claim that we should do this, I'm just saying that I could imagine this being a valid thing to do. But it might be a good idea to add a WARN_ON_ONCE() for now to find the users that are not being clever like this, they are just being stupid and wrong-headed, and sending IPI's to bogus CPU's not because they are doing really subtle smart stuff, but just because they never noticed how stupid they are..
Linus
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