Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()

[Paolo Bonzini]

On 14/07/21 12:41, Michael S. Tsirkin wrote:
> > --------------- 8< ---------------
> > From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Subject: [PATCH] eventfd: protect eventfd_wake_count with a local_lock
> >
> > eventfd_signal assumes that spin_lock_irqsave/spin_unlock_irqrestore is
> > non-preemptable and therefore increments and decrements the percpu
> > variable inside the critical section.
> >
> > This obviously does not fly with PREEMPT_RT.  If eventfd_signal is
> > preempted and an unrelated thread calls eventfd_signal, the result is
> > a spurious WARN.  To avoid this, protect the percpu variable with a
> > local_lock.
> >
> > Reported-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> > Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: He Zhe <zhe.he@xxxxxxxxxxxxx>
> > Cc: Jens Axboe <axboe@xxxxxxxxx>
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>
>
> Makes sense ...
>
> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>
> want to send this to the windriver guys so they can test?
> Here's the list from that thread:

I included He Zhe, but it should be enough for Daniel to test it.  The 
bug is quite obvious once you wear your PREEMPT_RT goggles.

Paolo