Re: [PATCH RFC] virtio: hint if callbacks surprisingly might sleep

Michael S. Tsirkin
2019-01-31T15:28:00Z

On Thu, Jan 31, 2019 at 01:53:14PM +0100, Cornelia Huck wrote:
> A virtio transport is free to implement some of the callbacks in
> virtio_config_ops in a matter that they cannot be called from
> atomic context (e.g. virtio-ccw, which maps a lot of the callbacks
> to channel I/O, which is an inherently asynchronous mechanism).
> This can be very surprising for developers using the much more
> common virtio-pci transport, just to find out that things break
> when used on s390.
>
> The documentation for virtio_config_ops now contains a comment
> explaining this, but it makes sense to add a might_sleep() annotation
> to various wrapper functions in the virtio core to avoid surprises
> later.
>
> Note that annotations are NOT added to two classes of calls:
> - direct calls from device drivers (all current callers should be
> fine, however)
> - calls which clearly won't be made from atomic context (such as
> those ultimately coming in via the driver core)
>
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>

Makes sense to me. I don't think we should push our luck in
this release though, better defer until the merge window.

> ---
>
> I think it is safe to add this now that the issues with the balloon
> have been fixed.
>
> Note that this is not bulletproof (nor is it inteded to be). The
> intention is to make it easier for people to catch problems earlier.
>
> ---
> drivers/virtio/virtio.c | 2 ++
> include/linux/virtio_config.h | 13 +++++++++++++
> 2 files changed, 15 insertions(+)
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 59e36ef4920f..98b30f54342c 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -161,6 +161,7 @@ EXPORT_SYMBOL_GPL(virtio_config_enable);
>
> void virtio_add_status(struct virtio_device *dev, unsigned int status)
> {
> + might_sleep();
> dev->config->set_status(dev, dev->config->get_status(dev) | status);
> }
> EXPORT_SYMBOL_GPL(virtio_add_status);
> @@ -170,6 +171,7 @@ int virtio_finalize_features(struct virtio_device *dev)
> int ret = dev->config->finalize_features(dev);
> unsigned status;
>
> + might_sleep();
> if (ret)
> return ret;
>
> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> index 987b6491b946..bb4cc4910750 100644
> --- a/include/linux/virtio_config.h
> +++ b/include/linux/virtio_config.h
> @@ -290,6 +290,7 @@ static inline __virtio64 cpu_to_virtio64(struct virtio_device *vdev, u64 val)
> /* Config space accessors. */
> #define virtio_cread(vdev, structname, member, ptr) \
> do { \
> + might_sleep(); \
> /* Must match the member's type, and be integer */ \
> if (!typecheck(typeof((((structname*)0)->member)), *(ptr))) \
> (*ptr) = 1; \
> @@ -319,6 +320,7 @@ static inline __virtio64 cpu_to_virtio64(struct virtio_device *vdev, u64 val)
> /* Config space accessors. */
> #define virtio_cwrite(vdev, structname, member, ptr) \
> do { \
> + might_sleep(); \
> /* Must match the member's type, and be integer */ \
> if (!typecheck(typeof((((structname*)0)->member)), *(ptr))) \
> BUG_ON((*ptr) == 1); \
> @@ -358,6 +360,7 @@ static inline void __virtio_cread_many(struct virtio_device *vdev,
> vdev->config->generation(vdev) : 0;
> int i;
>
> + might_sleep();
> do {
> old = gen;
>
> @@ -380,6 +383,8 @@ static inline void virtio_cread_bytes(struct virtio_device *vdev,
> static inline u8 virtio_cread8(struct virtio_device *vdev, unsigned int offset)
> {
> u8 ret;
> +
> + might_sleep();
> vdev->config->get(vdev, offset, &ret, sizeof(ret));
> return ret;
> }
> @@ -387,6 +392,7 @@ static inline u8 virtio_cread8(struct virtio_device *vdev, unsigned int offset)
> static inline void virtio_cwrite8(struct virtio_device *vdev,
> unsigned int offset, u8 val)
> {
> + might_sleep();
> vdev->config->set(vdev, offset, &val, sizeof(val));
> }
>
> @@ -394,6 +400,8 @@ static inline u16 virtio_cread16(struct virtio_device *vdev,
> unsigned int offset)
> {
> u16 ret;
> +
> + might_sleep();
> vdev->config->get(vdev, offset, &ret, sizeof(ret));
> return virtio16_to_cpu(vdev, (__force __virtio16)ret);
> }
> @@ -401,6 +409,7 @@ static inline u16 virtio_cread16(struct virtio_device *vdev,
> static inline void virtio_cwrite16(struct virtio_device *vdev,
> unsigned int offset, u16 val)
> {
> + might_sleep();
> val = (__force u16)cpu_to_virtio16(vdev, val);
> vdev->config->set(vdev, offset, &val, sizeof(val));
> }
> @@ -409,6 +418,8 @@ static inline u32 virtio_cread32(struct virtio_device *vdev,
> unsigned int offset)
> {
> u32 ret;
> +
> + might_sleep();
> vdev->config->get(vdev, offset, &ret, sizeof(ret));
> return virtio32_to_cpu(vdev, (__force __virtio32)ret);
> }
> @@ -416,6 +427,7 @@ static inline u32 virtio_cread32(struct virtio_device *vdev,
> static inline void virtio_cwrite32(struct virtio_device *vdev,
> unsigned int offset, u32 val)
> {
> + might_sleep();
> val = (__force u32)cpu_to_virtio32(vdev, val);
> vdev->config->set(vdev, offset, &val, sizeof(val));
> }
> @@ -431,6 +443,7 @@ static inline u64 virtio_cread64(struct virtio_device *vdev,
> static inline void virtio_cwrite64(struct virtio_device *vdev,
> unsigned int offset, u64 val)
> {
> + might_sleep();
> val = (__force u64)cpu_to_virtio64(vdev, val);
> vdev->config->set(vdev, offset, &val, sizeof(val));
> }
> --
> 2.17.2