Hi Will,
Thanks for your review.
On Tuesday 25 July 2017 06:55 PM, Will Deacon wrote:
On Fri, Jul 07, 2017 at 05:34:00PM +0530, Pratyush Anand wrote:
If an interrupt is generated between breakpoint and step handler then
step handler can not get correct step address. This situation can easily
be invoked by samples/hw_breakpoint/data_breakpoint.c. It can also be
reproduced if we insert any printk() statement or dump_stack() in perf
overflow_handler. So, it seems that perf is working fine just luckily.
If the CPU which is handling perf breakpoint handler receives any
interrupt then, perf step handler will not execute sanely.
This patch improves do_debug_exception() handling, which enforces now,
that exception handler function:
- should return 0 for any software breakpoint and hw
breakpoint/watchpoint handler if it does not expect a single step stage
- should return 1 if it expects single step.
- A single step handler should always return 0.
- All handler should return a -ve error in any other case.
Now, we can know in do_debug_exception() that whether a step exception
will be followed or not. If there will a step exception then disable
irq. Re-enable it after single step handling.
AFAICT, this is only a problem for kernel-mode breakpoints where we end up
stepping into the interrupt handler when trying to step over a breakpoint.
I think yes.
We'd probably be better off getting all users of kernel step (kprobes, kgdb
and perf) to run the step with irqs disabled,
That should be doable. We can easily manage all of them in
do_debug_exception() if individual brk handlers return correct value as per
the rule mentioned in the commit log of this patch.
I think, I can take care of kprobes and kgdb as well in next version of patch.
but I still have reservations
about that:
So, IIUC, you have concern about faulting of a instruction being stepped.
Since we will have a notion of *irq_en_needed*, so I think, if needed we can
re-enable interrupt in fault handler do_mem_abort().
Whats your opinion here?
http://lists.infradead.org/pipermail/linux-arm-kernel/2017-May/508066.html
http://lists.infradead.org/pipermail/linux-arm-kernel/2017-June/510814.html
Wouldn't it be better to follow kprobes/kgdb and have perf run the step with
irqs disabled?
--
Regards
Pratyush
