Re: frequent lockups in 3.18rc4

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Nov 21, 2014 at 11:25:06AM -0500, Tejun Heo wrote:
> Hello, Frederic.
> 
> On Fri, Nov 21, 2014 at 03:13:35PM +0100, Frederic Weisbecker wrote:
> ...
> > So when the issue arised 4 years ago, it was a problem only for NMIs.
> > Like Linus says: "what happens in NMI stays in NMI". Ok no that's not quite
> > what he says :-)  But NMIs happen to be a corner case for about everything
> > and it's sometimes better to fix things from NMI itself, or have an NMI
> > special case rather than grow the whole infrastructure in complexity to
> > support this very corner case.
> 
> I'm not familiar with the innards of fault handling, so can you please
> help me understand what may actually break?  Here are what I currently
> understand.
> 
> * Static percpu areas wouldn't trigger fault lazily.  Note that this
>   is not necessarily because the first percpu chunk which contains the
>   static area is embedded inside the kernel linear mapping.  Depending
>   on the memory layout and boot param, percpu allocator may choose to
>   map the first chunk in vmalloc space too; however, this still works
>   out fine because at that point there are no other page tables and
>   the PUD entries covering the first chunk is faulted in before other
>   pages tables are copied from the kernel one.
> 
> * NMI used to be a problem because vmalloc fault handler couldn't
>   safely nest inside NMI handler but this has been fixed since and it
>   should work fine from NMI handlers now.
> 
> * Function tracers are problematic because they may end up nesting
>   inside themselves through triggering a vmalloc fault while accessing
>   dynamic percpu memory area.  This may lead to recursive locking and
>   other surprises.
> 
> Are there other cases where the lazy vmalloc faults can break things?

I fear that enumerating and fix the existing issues won't be enough.
We can't find all the code sites out there which rely on not being
faulted.

The best would be to fix that from the percpu allocator itself, or vmalloc.