Re: [PATCH 3/4] KVM: introduce memory transaction semaphore

[Paolo Bonzini]

On 10/24/22 14:57, Emanuele Giuseppe Esposito wrote:
> Am 23/10/2022 um 19:50 schrieb Paolo Bonzini:
> > On 10/22/22 17:48, Emanuele Giuseppe Esposito wrote:
> > > +static DECLARE_RWSEM(memory_transaction);
> >
> > This cannot be global, it must be per-struct kvm.  Otherwise one VM can
> > keep the rwsem indefinitely while a second VM hangs in
> > KVM_KICK_ALL_RUNNING_VCPUS.
> >
> > It can also be changed to an SRCU (with the down_write+up_write sequence
> > changed to synchronize_srcu_expedited) which has similar characteristics
> > to your use of the rwsem.
>
> Makes sense, but why synchronize_srcu_expedited and not synchronize_srcu?

Because (thanks to the kick) you expect the grace period to end almost 
immediately, and synchronize_srcu() will slow down sensibly the changes 
to the memory map.

Paolo