On Fri, Jan 11, 2019 at 04:06:52PM +0800, Zhenzhong Duan wrote: > > On 2019/1/10 22:43, Waiman Long wrote: > > On 01/10/2019 03:02 AM, Zhenzhong Duan wrote: > > > Hi Maintainer, > > > > > > > > > There is a question confused me for days. Appreciate an answer. > > > > > > In below code, the comment says we never have more than 4 nested > > > contexts. > > > > > > What happen if debug and mce exceptions nest with the four, or we > > > ensure it never happen? > > > > > > > > > /* > > > * Per-CPU queue node structures; we can never have more than 4 nested > > > * contexts: task, softirq, hardirq, nmi. > > > * > > > * Exactly fits one 64-byte cacheline on a 64-bit architecture. > > > * > > > * PV doubles the storage and uses the second cacheline for PV state. > > > */ > > > static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]); > > > > > Yes, both debug and mce exceptions are some kind of NMIs. So > > theoretically, it is possible to have more than four. Are you aware of > > any debug and MCE exception handlers that need to take a spinlock for > > synchronization? > > Not for debug exception, for MCE exception handler I found below two: > > do_machine_check->mce_report_event->schedule_work > do_machine_check->force_sig->force_sig_info > > schedule_work() and force_sig_info() take spinlocks.
Boris, how can this be?