Re: [PATCH v3 02/22] x86,mce: Delete ist_begin_non_atomic()

[Peter Zijlstra]

On Wed, Feb 19, 2020 at 09:21:48AM -0800, Andy Lutomirski wrote:
> On Wed, Feb 19, 2020 at 9:13 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
> >
> > On Wed, Feb 19, 2020 at 03:47:26PM +0100, Peter Zijlstra wrote:
> > > Subject: Re: [PATCH v3 02/22] x86,mce: Delete ist_begin_non_atomic()
> >
> > x86/mce: ...
> >
> > > It is an abomination; and in prepration of removing the whole
> > > ist_enter() thing, it needs to go.
> > >
> > > Convert #MC over to using task_work_add() instead; it will run the
> > > same code slightly later, on the return to user path of the same
> > > exception.
> >
> > That's fine because the error happened in userspace.
> 
> Unless there is a signal pending and the signal setup code is about to
> hit the same failed memory.  I suppose we can just treat cases like
> this as "oh well, time to kill the whole system".
> 
> But we should genuinely agree that we're okay with deferring this handling.

It doesn't delay much. The moment it does that local_irq_enable() it's
subject to preemption, just like it is on the return to user path.

Do you really want to create code that unwinds enough of nmi_enter() to
get you to a preemptible context? *shudder*