On Fri, 16 Jul 2021 13:55:27 +0200 Paolo Bonzini wrote: >On 16/07/21 11:37, Hillf Danton wrote: >> On Fri, 16 Jul 2021 09:59:15 +0200 Paolo Bonzini wrote: >>> * the warning only occurs if preemption occurs during the >>> spin_lock_irqsave critical section (and therefore it can only occur in >>> PREEMPT_RT kernels) >> >> With that lock held, no waitqueue entry can be added on to the WQ - IOW no >> wakeup will go stray. >> >>> * the warning causes an early return 0 that messes up the VM's networking >> >> Is the messup due to the zero or wakeup? > >It's caused by the missing wakeup, i.e. eventfd_signal not really >signaling anything. There are two cases of write_seqcount_begin in x/virt/kvm/eventfd.c, and in kvm_irqfd_deassign() it is surrounded by spin_lock_irq(&kvm->irqfds.lock) that also protects irqfd_update(). What isnt clear is if the risk is zero that either case can be preempted by seqcount reader. That risk may end up with the livelock described in x/Documentation/locking/seqlock.rst. +A sequence counter write side critical section must never be preempted +or interrupted by read side sections. Otherwise the reader will spin for +the entire scheduler tick due to the odd sequence count value and the +interrupted writer. If that reader belongs to a real-time scheduling +class, it can spin forever and the kernel will livelock.
