[PATCH 2/3] vhost_task: Allow caller to omit handle_sigkill() callback

[Sean Christopherson <seanjc@google.com>]

Now that vhost_task provides an API to safely wake a task without relying
on the caller to react to signalas, make handle_sigkill() optional and
WARN if the "unsafe" vhost_task_wake() is used without hooking sigkill.
Requiring the user to react to sigkill adds no meaningful value, e.g. it
didn't help KVM do anything useful, and adding a sanity check in
vhost_task_wake() gives developers a hint as to what needs to be done in
response to sigkill.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 kernel/vhost_task.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
index 5aa8ddf88d01..e0ec6bfe61e6 100644
--- a/kernel/vhost_task.c
+++ b/kernel/vhost_task.c
@@ -59,7 +59,8 @@ static int vhost_task_fn(void *data)
 	 */
 	if (!test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags)) {
 		set_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags);
-		vtsk->handle_sigkill(vtsk->data);
+		if (vtsk->handle_sigkill)
+			vtsk->handle_sigkill(vtsk->data);
 	}
 	mutex_unlock(&vtsk->exit_mutex);
 	complete(&vtsk->exited);
@@ -81,6 +82,13 @@ static void __vhost_task_wake(struct vhost_task *vtsk)
  */
 void vhost_task_wake(struct vhost_task *vtsk)
 {
+	/*
+	 * Waking the task without taking exit_mutex is safe if and only if the
+	 * implementation hooks sigkill, as that's the only way the caller can
+	 * know if the task has exited prematurely due to a signal.
+	 */
+	WARN_ON_ONCE(!vtsk->handle_sigkill);
+
 	/*
 	 * Checking VHOST_TASK_FLAGS_KILLED can race with signal delivery, but
 	 * a race can only result in false negatives and this is just a sanity
-- 
2.51.0.261.g7ce5a0a67e-goog