Re: [PATCHv2] x86/xen: allow privcmd hypercalls to be preempted

[David Vrabel <david.vrabel@citrix.com>]

On 24/02/14 19:18, Konrad Rzeszutek Wilk wrote:
> On Thu, Feb 13, 2014 at 04:59:27PM +0000, David Vrabel wrote:
>> From: David Vrabel <david.vrabel@citrix.com>
>>
>> Hypercalls submitted by user space tools via the privcmd driver can
>> take a long time (potentially many 10s of seconds) if the hypercall
>> has many sub-operations.
>>
>> A fully preemptible kernel may deschedule such as task in any upcall
>> called from a hypercall continuation.
>>
>> However, in a kernel with voluntary or no preemption, hypercall
>> continuations in Xen allow event handlers to be run but the task
>> issuing the hypercall will not be descheduled until the hypercall is
>> complete and the ioctl returns to user space.  These long running
>> tasks may also trigger the kernel's soft lockup detection.
>>
>> Add xen_preemptible_hcall_begin() and xen_preemptible_hcall_end() to
>> bracket hypercalls that may be preempted.  Use these in the privcmd
>> driver.
>>
>> When returning from an upcall, call preempt_schedule_irq() if the
>> current task was within a preemptible hypercall.
>>
>> Since preempt_schedule_irq() can move the task to a different CPU,
>> clear and set xen_in_preemptible_hcall around the call.
>>
>> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
>> ---
>> Changes in v2:
>> - Use per-cpu variable to mark preemptible regions
>> - Call preempt_schedule_irq() from the correct place in
>>   xen_hypervisor_callback
> 
> 
> 12929 ERROR: "xen_in_preemptible_hcall" [drivers/xen/xen-privcmd.ko] undefined!

You have privcmd built as a module so xen_in_preemptible_hcall needs to
be exported.  I'll send a v3 shortly.

David