Messages in this thread |  | | From | Paolo Bonzini <> | Subject | Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() | Date | Mon, 19 Jul 2021 17:38:45 +0200 |
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On 18/07/21 14:42, Hillf Danton wrote: >> It's caused by the missing wakeup, i.e. eventfd_signal not really >> signaling anything. > > Can you please point me to the waiters in the mainline?
It's irqfd_wakeup.
> 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.
Since the introduction of seqcount_spinlock_t, the writers automatically disable preemption. This is definitely the right thing in this case where the seqcount writers are small enough, and the readers are hot enough, that using a local lock would be too heavyweight.
Without that, the livelock would be possible, though very unlikely. In practice seqcount updates should only happen while the producer is quiescent; and also the seqcount readers and writers will often be pinned to separate CPUs.
Paolo
> +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. >
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