Re: [Xen-devel] Strange PVM spinlock case revisited

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 11.02.2013 18:29, Ian Campbell wrote:

> An interesting hack^Wexperiment might be to make xen_poll_irq use a
> timeout and see if that unwedges things -- this would help confirm that
> the issue is on nested wakeup.
> 

So I did go forward and replaced xen_poll_irq by xen_poll_irq_timeout and it did
get rid of the hang. Though I think there is a big taint there. There was
only one other user of poll_irq_timeout in the kernel code. And that uses
"jiffies + <timeout>*HZ". But when I look at the Xen side in do_poll, that looks
like it is using timeout in a absolute "ns since boot" (of hv/dom0) way. Not
sure how that ever can work. The ns since boot in the guest clearly is always
behind the host (and jiffies isn't ns either).
Effectively I likely got rid of any wait time in the hypervisor and back to
mostly spinning. Which matches the experience that the test run never gets stuck
waiting for a timeout. That maybe proves the stacking is an issue but also is
likely a bit too aggressive in not having any... :/

I will try to think of some better way. Not sure the thinking is realistic but
maybe that could happen:

xen_spin_lock_slow(a)
  ...
  enables irq and upcalls are pending
    upcall processing wants lock b
    xen_spin_lock_slow(b)
                    --- just before replacing lock_spinners ---
                                                   xen_spin_unlock_slow(a)
                                                   finds other vcpu, triggers
                                                     IRQ
    lock b is top spinner
    going into poll_irq
    poll_irq returns
    lock a gets restored
    so maybe no spinners on b
    dropping out to xen_spin_lock
                                                  unlock of b not finding any
                                                  spinners
    lock b acquired

That way the irq for lock a maybe get lost...

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