Subject: Re: [PATCH] virtio_pci_modern: Use GFP_ATOMIC with spin_lock_irqsave held in virtqueue_exec_admin_cmd()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 13 Apr 2026 03:45:20 -0400

On Mon, Apr 13, 2026 at 03:22:49PM +0800, Jinhui Guo wrote:
> virtqueue_exec_admin_cmd() holds admin_vq->lock with spin_lock_irqsave(),
> which disables interrupts.  Using GFP_KERNEL inside this critical section
> is unsafe because kmalloc() may sleep, leading to potential deadlocks or
> scheduling violations.
> 
> Switch to GFP_ATOMIC to ensure the allocation is non-blocking.
> 
> Fixes: 4c3b54af907e ("virtio_pci_modern: use completion instead of busy loop to wait on admin cmd result")
> Cc: [email-masked]
> Signed-off-by: Jinhui Guo <[email-masked]>
> ---
>  drivers/virtio/virtio_pci_modern.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
> index 6d8ae2a6a8ca..db8e4f88b749 100644
> --- a/drivers/virtio/virtio_pci_modern.c
> +++ b/drivers/virtio/virtio_pci_modern.c
> @@ -101,7 +101,7 @@ static int virtqueue_exec_admin_cmd(struct virtio_pci_admin_vq *admin_vq,
>  		return -EIO;
>  
>  	spin_lock_irqsave(&admin_vq->lock, flags);
> -	ret = virtqueue_add_sgs(vq, sgs, out_num, in_num, cmd, GFP_KERNEL);
> +	ret = virtqueue_add_sgs(vq, sgs, out_num, in_num, cmd, GFP_ATOMIC);
>  	if (ret < 0) {
>  		if (ret == -ENOSPC) {
>  			spin_unlock_irqrestore(&admin_vq->lock, flags);

GFP_ATOMIC allocations can and will fail. If using them, one must
retry, not just propagate failures.
Or just switch admin_vq->lock to a mutex?

> -- 
> 2.20.1