Re: frequent lockups in 3.18rc4

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Nov 21, 2014 at 12:01:51PM -0500, Steven Rostedt wrote:
> On Fri, Nov 21, 2014 at 11:25:06AM -0500, Tejun Heo wrote:
> > 
> > * 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.
> 
> That sounds correct.
> 
> > 
> > * 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.
> 
> Right. Of course "should work fine" does not excatly mean "will work fine".
> 
> 
> > 
> > * 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.
> 
> The function tracer infrastructure now has a recursive check that happens
> rather early in the call. Unless the registered OPS specifically states
> it handles recursions (FTRACE_OPS_FL_RECUSION_SAFE), ftrace will add the
> necessary recursion checks. If a registered OPS lies about being recusion
> safe, well we can't stop suicide.

Same if the recursion state is based on per cpu memory.

> 
> Looking at kernel/trace/trace_functions.c: function_trace_call() which is
> registered with RECURSION_SAFE, I see that the recursion check is done
> before the per_cpu_ptr() call to the dynamically allocated per_cpu data.
> 
> It looks OK, but...
> 
> Oh! but if we trace the page fault handler, and we fault here too
> we just nuked the cr2 register. Not good.

If we fault in the page fault handler, we double fault and apparently
recovering from that isn't quite expected anyway.