On 07.08.20 11:50, Marco Elver wrote:

On Fri, Aug 07, 2020 at 11:24AM +0200, Jürgen Groß wrote:

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On 07.08.20 11:01, Marco Elver wrote:

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On Thu, 6 Aug 2020 at 18:06, Marco Elver [off-list ref] wrote:

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On Thu, 6 Aug 2020 at 15:17, Marco Elver [off-list ref] wrote:

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On Thu, Aug 06, 2020 at 01:32PM +0200, peterz@infradead.org wrote:

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On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco Elver wrote:

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Testing my hypothesis that raw then nested non-raw
local_irq_save/restore() breaks IRQ state tracking -- see the reproducer
below. This is at least 1 case I can think of that we're bound to hit.

...

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/me goes ponder things...

How's something like this then?

---
   include/linux/sched.h |  3 ---
   kernel/kcsan/core.c   | 62 ++++++++++++++++++++++++++++++++++++---------------
   2 files changed, 44 insertions(+), 21 deletions(-)

Thank you! That approach seems to pass syzbot (also with
CONFIG_PARAVIRT) and kcsan-test tests.

I had to modify it some, so that report.c's use of the restore logic
works and not mess up the IRQ trace printed on KCSAN reports (with
CONFIG_KCSAN_VERBOSE).

I still need to fully convince myself all is well now and we don't end
up with more fixes. :-) If it passes further testing, I'll send it as a
real patch (I want to add you as Co-developed-by, but would need your
Signed-off-by for the code you pasted, I think.)

I let it run on syzbot through the night, and it's fine without
PARAVIRT (see below). I have sent the patch (need your Signed-off-by
as it's based on your code, thank you!):
https://lkml.kernel.org/r/20200807090031.3506555-1-elver@google.com

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With CONFIG_PARAVIRT=y (without the notrace->noinstr patch), I still
get lockdep DEBUG_LOCKS_WARN_ON(!lockdep_hardirqs_enabled()), although
it takes longer for syzbot to hit them. But I think that's expected
because we can still get the recursion that I pointed out, and will
need that patch.

Never mind, I get these warnings even if I don't turn on KCSAN
(CONFIG_KCSAN=n). Something else is going on with PARAVIRT=y that
throws off IRQ state tracking. :-/

What are the settings of CONFIG_PARAVIRT_XXL and
CONFIG_PARAVIRT_SPINLOCKS in this case?

I attached a config.

	$> grep PARAVIRT .config
	CONFIG_PARAVIRT=y
	CONFIG_PARAVIRT_XXL=y
	# CONFIG_PARAVIRT_DEBUG is not set
	CONFIG_PARAVIRT_SPINLOCKS=y
	# CONFIG_PARAVIRT_TIME_ACCOUNTING is not set
	CONFIG_PARAVIRT_CLOCK=y

Anything special I need to do to reproduce the problem? Or would you be
willing to do some more rounds with different config settings?

I think CONFIG_PARAVIRT_XXL shouldn't matter, but I'm not completely
sure about that. CONFIG_PARAVIRT_SPINLOCKS would be my primary suspect.


Juergen
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