Messages in this thread | | | Date | Tue, 15 May 2018 14:20:42 +0900 | From | Sergey Senozhatsky <> | Subject | Re: printk feature for syzbot? |
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Hello,
On (05/11/18 09:37), Steven Rostedt wrote: > > On (05/11/18 11:17), Dmitry Vyukov wrote: > > > > > > 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.
> > 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
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