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

Paolo Bonzini
2022-10-25T10:08:48Z

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