Re: [PATCH 07/10] MCE: replace mce.c use of TIF_MCE_NOTIFY with user_return_notifier

[Tony Luck <tony.luck@intel.com>]

2011/6/10 Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>:
> Now I'm reconsidering the MCE event notification mechanism.
> One of something nervous is whether it is really required to process
> "_AO" memory poisoning (i.e. mce_process_ring()) here in a process
> context that unfortunately interrupted by MCE (or preempted after that).
> I'm uncertain how long walking though the task_list for unmap will takes,
> and not sure it is acceptable if the unlucky thread is a kind of latency
> sensitive...
>
> If we can move mce_process_ring() to worker thread completely, what
> we have to do will be:
>  1) from NMI context, request non-NMI context by irq_work()
>  2) from (irq) context, wake up loggers and schedule work if required
>  3) from worker thread, process "_AO" memory poisoning etc.
>
> So now question is why user_return_notifier is needed here.
> Is it just an alternative of irq_work() for !LOCAL_APIC ?

I switched this notification over from old TIF_MCE_NOTIFY to
using user_return_notifier to preserve existing semantics, and
free up that TIF bit for the task notifier for the action required case.

You are right that we should have some better method - the
shot-gun approach of hitting every task on every cpu in the hope
that one of them will run our code soon sounds like overkill.
Using some NMI-saf method to wake a single worker thread
sounds a good idea for a subsequent clean up.

-Tony