Implement UMCG server/worker API.

This is an early RFC patch - the code seems working, but
more testing is needed. Gaps I plan to address before this
is ready for a detailed review:

- preemption/interrupt handling;
- better documentation/comments;
- tracing;
- additional testing;
- corner cases like abnormal process/task termination;
- in some cases where I kill the task (umcg_segv), returning
an error may be more appropriate.

All in all, please focus more on the high-level approach
and less on things like variable names, (doc) comments, or indentation.

Signed-off-by: Peter Oskolkov <redacted>
---
 include/linux/mm_types.h |   5 +
 include/linux/syscalls.h |   5 +
 kernel/fork.c            |  11 +
 kernel/sched/core.c      |  11 +
 kernel/sched/umcg.c      | 764 ++++++++++++++++++++++++++++++++++++++-
 kernel/sched/umcg.h      |  54 +++
 mm/init-mm.c             |   4 +
 7 files changed, 845 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6613b26a8894..5ca7b7d55775 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -562,6 +562,11 @@ struct mm_struct {
 #ifdef CONFIG_IOMMU_SUPPORT
 		u32 pasid;
 #endif
+
+#ifdef CONFIG_UMCG
+	spinlock_t umcg_lock;
+	struct list_head umcg_groups;
+#endif
 	} __randomize_layout;
 
 	/*

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 15de3e34ccee..2781659daaf1 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -1059,6 +1059,11 @@ asmlinkage long umcg_wait(u32 flags, const struct __kernel_timespec __user *time
 asmlinkage long umcg_wake(u32 flags, u32 next_tid);
 asmlinkage long umcg_swap(u32 wake_flags, u32 next_tid, u32 wait_flags,
 				const struct __kernel_timespec __user *timeout);
+asmlinkage long umcg_create_group(u32 api_version, u64, flags);
+asmlinkage long umcg_destroy_group(u32 group_id);
+asmlinkage long umcg_poll_worker(u32 flags, struct umcg_task __user **ut);
+asmlinkage long umcg_run_worker(u32 flags, u32 worker_tid,
+		struct umcg_task __user **ut);
 
 /*
  * Architecture-specific system calls

diff --git a/kernel/fork.c b/kernel/fork.c
index ace4631b5b54..3a2a7950df8e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c

@@ -1026,6 +1026,10 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	seqcount_init(&mm->write_protect_seq);
 	mmap_init_lock(mm);
 	INIT_LIST_HEAD(&mm->mmlist);
+#ifdef CONFIG_UMCG
+	spin_lock_init(&mm->umcg_lock);
+	INIT_LIST_HEAD(&mm->umcg_groups);
+#endif
 	mm->core_state = NULL;
 	mm_pgtables_bytes_init(mm);
 	mm->map_count = 0;

@@ -1102,6 +1106,13 @@ static inline void __mmput(struct mm_struct *mm)
 		list_del(&mm->mmlist);
 		spin_unlock(&mmlist_lock);
 	}
+#ifdef CONFIG_UMCG
+	if (!list_empty(&mm->umcg_groups)) {
+		spin_lock(&mm->umcg_lock);
+		list_del(&mm->umcg_groups);