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

[Cornelia Huck]

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@xxxxxxxxxx>
---

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