Re: [Xen-devel] [PATCH v4] x86/cpu: Sync any remaining RCU callbacks before CPU up/down

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.03.2020 19:06, Igor Druzhinin wrote:
> During CPU down operation RCU callbacks are scheduled to finish
> off some actions later as soon as CPU is fully dead (the same applies
> to CPU up operation in case error path is taken). If in the same grace
> period another CPU up operation is performed on the same CPU, RCU callback
> will be called later on a CPU in a potentially wrong (already up again
> instead of still being down) state leading to eventual state inconsistency
> and/or crash.
> 
> In order to avoid it - flush RCU callbacks explicitly before starting the
> next CPU up/down operation.
> 
> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> ---
> This got discovered trying to resume PV shim with multiple vCPUs on AMD
> machine (where park_offline_cpus == 0). RCU callback responsible for
> freeing percpu area on CPU offline got finally called after CPU went
> online again as the guest performed regular vCPU offline/online operations
> on resume.
> 
> Note: this patch requires RCU series v4 from Juergen to be applied -
> https://lists.xenproject.org/archives/html/xen-devel/2020-03/msg00668.html

I was about to apply the patch yesterday (I think) when I stumbled
across this note. Is this actually still true? If so, would you
mind helping me see the dependency?

Jan

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