[PATCH v2 1/4] task: Add a count of task rcu users

[Eric W. Biederman]

Add a count of the number of rcu users (currently 1) of the task
struct so that we can later add the scheduler case and get rid of the
very subtle task_rcu_dereference, and just use rcu_dereference.

As suggested by Oleg have the count overlap rcu_head so that no
additional space in task_struct is required.

Inspired-by: Linus Torvalds <torva...@linux-foundation.org>
Inspired-by: Oleg Nesterov <o...@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebied...@xmission.com>
---
 include/linux/sched.h      | 5 ++++-
 include/linux/sched/task.h | 1 +
 kernel/exit.c              | 7 ++++++-
 kernel/fork.c              | 7 +++----
 4 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9f51932bd543..99a4518b9b17 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1142,7 +1142,10 @@ struct task_struct {
 
        struct tlbflush_unmap_batch     tlb_ubc;
 
-       struct rcu_head                 rcu;
+       union {
+               refcount_t              rcu_users;
+               struct rcu_head         rcu;
+       };
 
        /* Cache last used pipe for splice(): */
        struct pipe_inode_info          *splice_pipe;
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 0497091e40c1..4c44c37236b2 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -116,6 +116,7 @@ static inline void put_task_struct(struct task_struct *t)
 }
 
 struct task_struct *task_rcu_dereference(struct task_struct **ptask);
+void put_task_struct_rcu_user(struct task_struct *task);
 
 #ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT
 extern int arch_task_struct_size __read_mostly;
diff --git a/kernel/exit.c b/kernel/exit.c
index 5b4a5dcce8f8..2e259286f4e7 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -182,6 +182,11 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
        put_task_struct(tsk);
 }
 
+void put_task_struct_rcu_user(struct task_struct *task)
+{
+       if (refcount_dec_and_test(&task->rcu_users))
+               call_rcu(&task->rcu, delayed_put_task_struct);
+}
 
 void release_task(struct task_struct *p)
 {
@@ -222,7 +227,7 @@ void release_task(struct task_struct *p)
 
        write_unlock_irq(&tasklist_lock);
        release_thread(p);
-       call_rcu(&p->rcu, delayed_put_task_struct);
+       put_task_struct_rcu_user(p);
 
        p = leader;
        if (unlikely(zap_leader))
diff --git a/kernel/fork.c b/kernel/fork.c
index 2852d0e76ea3..9f04741d5c70 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -900,10 +900,9 @@ static struct task_struct *dup_task_struct(struct 
task_struct *orig, int node)
        if (orig->cpus_ptr == &orig->cpus_mask)
                tsk->cpus_ptr = &tsk->cpus_mask;
 
-       /*
-        * One for us, one for whoever does the "release_task()" (usually
-        * parent)
-        */
+       /* One for the user space visible state that goes away when reaped. */
+       refcount_set(&tsk->rcu_users, 1);
+       /* One for the rcu users, and one for the scheduler */
        refcount_set(&tsk->usage, 2);
 #ifdef CONFIG_BLK_DEV_IO_TRACE
        tsk->btrace_seq = 0;
-- 
2.21.0.dirty