HyperV/vmm: improve locking of hv_ring_buffer

[howard0su_gmail.com]

Summary

1. convert spin lock to def lock
2. reading always happen in single thread, remove lock protection

[sepherosa_gmail.com]

Seems reasonable to me

[adrian]

sys/dev/hyperv/vmbus/hv_ring_buffer.c

357

So this is nit picking, but do yo need memory barriers here if you intend to peek? the lock ends up being effectively a memory barrier..

[howard0su_gmail.com]

No. Since read/peek are happened in single thread, no mb is needed. Peek is just a normal read except it copies less data and doesn't advance the read position.

[sepherosa_gmail.com]

I just double checked the code. It is not MPSAFE to process the recv ring w/o lock, e.g. storvsc_poll(), which could be called in another thread. I think the spinlock -> default mtx change should be fine.

[sepherosa_gmail.com]

In D5164#115390, @sepherosa_gmail.com wrote:

I just double checked the code. It is not MPSAFE to process the recv ring w/o lock, e.g. storvsc_poll(), which could be called in another thread. I think the spinlock -> default mtx change should be fine.

I think we could pass a flag down to the recv ring read/peek, so that we could do lockless read for other drivers.