Messages in this thread | | | Subject | Re: [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt | From | Lai Jiangshan <> | Date | Fri, 1 Nov 2019 10:29:41 +0800 |
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On 2019/11/1 8:19 上午, Boqun Feng wrote: > On Thu, Oct 31, 2019 at 11:52:58AM -0700, Paul E. McKenney wrote: >> On Thu, Oct 31, 2019 at 11:14:23PM +0800, Lai Jiangshan wrote: >>> >>> >>> On 2019/10/31 10:31 下午, Paul E. McKenney wrote: >>>> On Thu, Oct 31, 2019 at 06:47:31AM -0700, Paul E. McKenney wrote: >>>>> On Thu, Oct 31, 2019 at 10:07:57AM +0000, Lai Jiangshan wrote: >>>>>> These is a possible bug (although which I can't triger yet) >>>>>> since 2015 8203d6d0ee78 >>>>>> (rcu: Use single-stage IPI algorithm for RCU expedited grace period) >>>>>> >>>>>> rcu_read_unlock() >>>>>> ->rcu_read_lock_nesting = -RCU_NEST_BIAS; >>>>>> interrupt(); // before or after rcu_read_unlock_special() >>>>>> rcu_read_lock() >>>>>> fetch some rcu protected pointers >>>>>> // exp GP starts in other cpu. >>>>>> some works >>>>>> NESTED interrupt for rcu_exp_handler(); >>>> >>>> Also, which platforms support nested interrupts? Last I knew, this was >>>> prohibited. >>>> >>>>>> report exp qs! BUG! >>>>> >>>>> Why would a quiescent state for the expedited grace period be reported >>>>> here? This CPU is still in an RCU read-side critical section, isn't it? >>>> >>>> And I now see what you were getting at here. Yes, the current code >>>> assumes that interrupt-disabled regions, like hardware interrupt >>>> handlers, cannot be interrupted. But if interrupt-disabled regions such >>>> as hardware interrupt handlers can be interrupted (as opposed to being >>>> NMIed), wouldn't that break a whole lot of stuff all over the place in >>>> the kernel? So that sounds like an arch bug to me. >>> >>> I don't know when I started always assuming hardware interrupt >>> handler can be nested by (other) interrupt. I can't find any >>> documents say Linux don't allow nested interrupt handler. >>> Google search suggests the opposite. > > FWIW, there is a LWN article talking about we disallow interrupt nesting > in *most* cases: > > https://lwn.net/Articles/380931/
Much thanks for the information!
> > , that's unless a interrupt handler explicitly calls > local_irq_enable_in_hardirq(), it remains irq disabled, which means no > nesting interrupt allowed. > Even so the problem here will be fixed by patch7/8.
> >> >> The results I am seeing look to be talking about threaded interrupt >> handlers, which indeed can be interrupted by hardware interrupts. As can >> softirq handlers. But these are not examples of a hardware interrupt >> handler being interrupted by another hardware interrupt. For that to >> work reasonably, something like a system priority level is required, >> as in the old DYNIX/ptx kernel, or, going even farther back, DEC's RT-11. >> >>> grep -rIni nested Documentation/memory-barriers.txt Documentation/x86/ >>> It still have some words about nested interrupt handler. >> >> Some hardware does not differentiate between interrupts and exceptions, >> for example, an illegal-instruction trap within an interrupt handler >> might look in some ways like a nested interrupt. >> >>> The whole patchset doesn't depend on this patch, and actually >>> it is reverted later in the patchset. Dropping this patch >>> can be an option for next round. >> >> Sounds like a plan! >> >> Thanx, Paul >> > [...] >
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