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/ , that's unless a interrupt handler explicitly calls local_irq_enable_in_hardirq(), it remains irq disabled, which means no nesting interrupt allowed. Regards, Boqun > > 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 > [...]
