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SubjectRe: frequent lockups in 3.18rc4
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.


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