Fix vcam streaming lifecycle correctness in videobuf

[davidzwei]

Two correctness fixes in videobuf.c for the streaming lifecycle:

• vcam_start_streaming(): kthread_create() returns ERR_PTR(-ENOMEM)
  on failure, not NULL. The previous !ptr check evaluates to false
  for any ERR_PTR value since it is a non-zero pointer, leaving
  sub_thr_id holding an invalid address. Replace with IS_ERR() and
  reset sub_thr_id to NULL on failure so vcam_stop_streaming()
  skips the kthread_stop() call safely.

• vcam_stop_streaming(): vb2_buffer_done() must not be called in
  atomic context as it internally invokes wake_up() on the done wait
  queue. Use list_splice_init() to transfer all active buffers to a
  temporary local list in O(1) while holding the spinlock, then release
  the lock before iterating and calling vb2_buffer_done(). This also
  minimises the critical section length.

---

Summary by cubic

Fixes vcam streaming lifecycle issues in videobuf.c to avoid crashes and stuck buffers. Improves correctness when starting and stopping streaming.

• Bug Fixes
  • Use IS_ERR() for kthread_create() and reset sub_thr_id to NULL on failure so vcam_stop_streaming() safely skips kthread_stop().
  • Move vb2_buffer_done() out of the spinlock: list_splice_init() active buffers to a temp list under lock, then complete them unlocked with VB2_BUF_STATE_ERROR.

^{Written for commit 10ead39. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}