On 01/11/2019 12:06 AM, 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. > -- > Thanks > Zhenzhong
The comment for do_machine_scheck() has already state that: * This is executed in NMI context not subject to normal locking rules. This * implies that most kernel services cannot be safely used. Don't even * think about putting a printk in there! So even if it doesn't exceed the MAX_NODES limit, it could hit deadlock and other kind of locking hazards. So supporting MCE is not a reason strong enough to extend MAX_NODES. In hindsight, we should have added a "BUG_ON(idx >= MAX_NODES);" check to avoid silent corruption because of that issue. Thanks, Longman