On 06.08.24 10:18, Dragos Tatulea wrote:

(Re-sending. I messed up the previous message, sorry about that.)

On 06.08.24 04:57, Jason Wang wrote:

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On Mon, Aug 5, 2024 at 11:59 PM Dragos Tatulea [off-list ref] wrote:

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On 05.08.24 05:17, Jason Wang wrote:

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On Fri, Aug 2, 2024 at 2:51 PM Dragos Tatulea [off-list ref] wrote:

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On Fri, 2024-08-02 at 11:29 +0800, Jason Wang wrote:

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On Thu, Aug 1, 2024 at 11:38 PM Dragos Tatulea [off-list ref] wrote:

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The following workflow triggers the crash referenced below:

1) vhost_vdpa_unsetup_vq_irq() unregisters the irq bypass producer
   but the producer->token is still valid.
2) vq context gets released and reassigned to another vq.

Just to make sure I understand here, which structure is referred to as
"vq context" here? I guess it's not call_ctx as it is a part of the vq
itself.

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3) That other vq registers it's producer with the same vq context
   pointer as token in vhost_vdpa_setup_vq_irq().

Or did you mean when a single eventfd is shared among different vqs?

Yes, that's what I mean: vq->call_ctx.ctx which is a eventfd_ctx.

But I don't think it's shared in this case, only that the old eventfd_ctx value
is lingering in producer->token. And this old eventfd_ctx is assigned now to
another vq.

Just to make sure I understand the issue. The eventfd_ctx should be
still valid until a new VHOST_SET_VRING_CALL().

I think it's not about the validity of the eventfd_ctx. More about
the lingering ctx value of the producer after vhost_vdpa_unsetup_vq_irq().

Probably, but

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That value is the eventfd ctx, but it could be anything else really...

I mean we hold a refcnt of the eventfd so it should be valid until the
next set_vring_call() or vhost_dev_cleanup().

But I do spot some possible issue:

1) We swap and assign new ctx in vhost_vring_ioctl():

                swap(ctx, vq->call_ctx.ctx);

2) and old ctx will be put there as well:

                if (!IS_ERR_OR_NULL(ctx))
                        eventfd_ctx_put(ctx);

3) but in vdpa, we try to unregister the producer with the new token:

static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
                           void __user *argp)
{
...
        r = vhost_vring_ioctl(&v->vdev, cmd, argp);
...
        switch (cmd) {
...
        case VHOST_SET_VRING_CALL:
                if (vq->call_ctx.ctx) {
                        cb.callback = vhost_vdpa_virtqueue_cb;
                        cb.private = vq;
                        cb.trigger = vq->call_ctx.ctx;
                } else {
                        cb.callback = NULL;
                        cb.private = NULL;
                        cb.trigger = NULL;
                }
                ops->set_vq_cb(vdpa, idx, &cb);
                vhost_vdpa_setup_vq_irq(v, idx);

in vhost_vdpa_setup_vq_irq() we had:

        irq_bypass_unregister_producer(&vq->call_ctx.producer);

here the producer->token still points to the old one...

Is this what you have seen?

Yup. That is the issue. The unregister already happened at
vhost_vdpa_unsetup_vq_irq(). So this second unregister will
work on an already unregistered element due to the token still
being set.

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I may miss something but the only way to assign exactly the same
eventfd_ctx value to another vq is where the guest tries to share the
MSI-X vector among virtqueues, then qemu will use a single eventfd as
the callback for multiple virtqueues. If this is true:

I don't think this is the case. I see the issue happening when running qemu vdpa
live migration tests on the same host. From a vdpa device it's basically a device
starting on a VM over and over.

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For bypass registering, only the first registering can succeed as the
following registering will fail because the irq bypass manager already
had exactly the same producer token.
For registering, all unregistering can succeed:

1) the first unregistering will do the real job that unregister the token
2) the following unregistering will do nothing by iterating the
producer token list without finding a match one

Maybe you can show me the userspace behaviour (ioctls) when you see this?

Sure, what would you need? qemu traces?

Yes, that would be helpful.

Will try to get them.

As the traces are quite large (~5MB), I uploaded them in this location [0].
I used the following qemu traces:
--trace vhost_vdpa* --trace virtio_net_handle*

[0] https://drive.google.com/file/d/1XyXYyockJ_O7zMgI7vot6AxYjze9Ljju/view?usp=sharing

Thanks,
Dragos