On Fri, Feb 20, 2015 at 07:03:14PM +0100, Jan Kiszka wrote:
> Hi Gilles,
>
> analyzing a lockdep warning on 3.16 with I-pipe enabled, I dug deeper
> into the hard and virtual interrupt state management during exception
> handling on ARM. I think there are several issues:
>
> - ipipe_fault_entry should not fiddle with the root irq state if run
> over head, only when invoked over root.
> - ipipe_fault_exit must not change the root state unless we entered over
> head and are about to leave over root - see x86. The current code may
> keep root incorrectly stalled after an exception, though this will
> probably be fixed up again in practice quickly.
> - do_sect_fault is only called by do_DataAbort and do_PrefetchAbort,
> in both cases already wrapped in ipipe_fault_entry/exit, thus it
> shouldn't invoke them once again.
>
> Room for optimization:
> - ipipe_fault_entry is always called with hard IRQs off from
> do_page_fault and do_translation_fault. I suspect this applies to the
> remaining callers (do_DataAbort and do_PrefetchAbort ) as well. Thus
> the hard IRQ state is actually known at compile time, right?
>
> I can hack up patches, but I'd like to confirm first that I'm not
> missing anything subtle or ARM-specific here.
Just to explain the original hack.
Some time ago, the faults handlers were executed irqs on ARM. The
irqs were enabled in entry.S before executing the handlers.
At some point, this was removed in entry.S and fault handlers
started to be executed irqs off. On ARM, all faults relax to be
handled in secondary mode, actually there is an exception, the FPU,
but it goes through a completely different path which had always
been executed irqs off until recently where the irqs are reenabled
when accessing user-space to be able to handle faults without
lockups.
My concern was that the code thus executed could have assertion
about the root domain being stalled which would be fail, so I added
code which stalled root and enabled hardware irqs on fault entry and
unstalled root and disabled hardware irqs on fault exit (which
always happen on root domain). This should have worked even if a fault
had happened to be handled in head domain, because then the
operation would have been a nop (simply stall/then unstall).
But Philippe found this dumb approach to fail when working on LPAE,
IIRC. IIRC, namely, if the root domain happens to be stalled when
entering a fault over head domain, it would end up unstalled after
the operation. So, I believe the code he added saves the stall state
on fault entry and restores it on fault exit. I have checked
Philippe's code details at the time and did not find anything wrong.
--
Gilles.
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