Hello,

On (05/11/18 09:37), Steven Rostedt wrote:

quoted

On (05/11/18 11:17), Dmitry Vyukov wrote:

quoted

From what I see, it seems that interrupts can be nested:  

Hm, I thought that in general IRQ handlers run with local IRQs
disabled on CPU. So, generally, IRQs don't nest. Was I wrong?
NMIs can nest, that's true; but I thought that at least IRQs
don't.

We normally don't run nested interrupts, although as the comment in
preempt.h says:

 * The hardirq count could in theory be the same as the number of
 * interrupts in the system, but we run all interrupt handlers with
 * interrupts disabled, so we cannot have nesting interrupts. Though
 * there are a few palaeontologic drivers which reenable interrupts in
 * the handler, so we need more than one bit here.

And no, NMI handlers do not nest. Yes, we deal with nested NMIs, but in
those cases, we just set a bit as a latch, and return, and when the
first NMI is complete, it checks that bit and if it is set, it executes
another NMI handler.

Good to know!
I thought that NMI can nest in some weird cases, like a breakpoint from
NMI. This must be super tricky, given that nested NMI will corrupt the
stack of the previous NMI, etc. Anyway.

quoted

Well, hm. __irq_enter() does preempt_count_add(HARDIRQ_OFFSET) and
__irq_exit() does preempt_count_sub(HARDIRQ_OFFSET). So, technically,
you can store

	preempt_count() & HARDIRQ_MASK
	preempt_count() & SOFTIRQ_MASK
	preempt_count() & NMI_MASK

[..]

I handle nesting of different contexts in the ftrace ring buffer using
the preempt count. See trace_recursive_lock/unlock() in
kernel/trace/ring_buffer.c.

Thanks. So you are also checking the preempt_count().

	-ss