Re: [PATCH 1/2] KVM: PPC: Book3S HV: Fix race in starting secondary threads

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Sep 03, 2015 at 03:20:50PM +1000, Paul Mackerras wrote:
> From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
> 
> The current dynamic micro-threading code has a race due to which a
> secondary thread naps when it is supposed to be running a vcpu. As a
> side effect of this, on a guest exit, the primary thread in
> kvmppc_wait_for_nap() finds that this secondary thread hasn't cleared
> its vcore pointer. This results in "CPU X seems to be stuck!"
> warnings.
> 
> The race is possible since the primary thread on exiting the guests
> only waits for all the secondaries to clear its vcore pointer. It
> subsequently expects the secondary threads to enter nap while it
> unsplits the core. A secondary thread which hasn't yet entered the nap
> will loop in kvm_no_guest until its vcore pointer and the do_nap flag
> are unset. Once the core has been unsplit, a new vcpu thread can grab
> the core and set the do_nap flag *before* setting the vcore pointers
> of the secondary. As a result, the secondary thread will now enter nap
> via kvm_unsplit_nap instead of running the guest vcpu.
> 
> Fix this by setting the do_nap flag after setting the vcore pointer in
> the PACA of the secondary in kvmppc_run_core. Also, ensure that a
> secondary thread doesn't nap in kvm_unsplit_nap when the vcore pointer
> in its PACA struct is set.
> 
> Fixes: b4deba5c41e9
> Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> Signed-off-by: Paul Mackerras <paulus@samba.org>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
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