linux-kernel - Re: [PATCH v9 5/6] vduse: add vq group asid support

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Message-ID: <CAJaqyWcY=yXnh+ZT7qJkXK4GbtUmwmfhWUXhdi=c38ifeaaPbQ@mail.gmail.com>
Date: Mon, 17 Nov 2025 13:16:23 +0100
From: Eugenio Perez Martin <eperezma@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>, Laurent Vivier <lvivier@...hat.com>, 
	virtualization@...ts.linux.dev, Maxime Coquelin <mcoqueli@...hat.com>, 
	Cindy Lu <lulu@...hat.com>, linux-kernel@...r.kernel.org, 
	Yongji Xie <xieyongji@...edance.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Stefano Garzarella <sgarzare@...hat.com>
Subject: Re: [PATCH v9 5/6] vduse: add vq group asid support

On Mon, Nov 17, 2025 at 11:50 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Fri, Nov 14, 2025 at 12:25:03PM +0100, Eugenio Perez Martin wrote:
> > RCU is protecting that the address space pointer of the vq group is
> > not modified concurrently with the access. Ideally, this should be a
> > full lock, but just making sure that all accesses from the reader are
> > coherent is enough. Userspace should expect nothing if it uses the map
> > and modifies the vq group ASID at the same time anyway, but the kernel
> > needs to be sure that it does not see intermediate states. TBH, we
> > could move to a READ_ONCE / WRITE_ONCE, would that be more clear?
>
> generally rcu itself does not need ONCE macros.
> these are for funky lockless things, and rcu can be
> seen as a kind of lock, after all.
>

Right, I meant to replace RCU by READ_ONCE / WRITE ONCE. But I also prefer RCU.