tasks: Add a count of task RCU users

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 <torvalds@linux-foundation.org>
Inspired-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/87woebdplt.fsf_-_@x220.int.ebiederm.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 3 insertions, 4 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 1d1cd06edbc1..7eefe338d7a2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -902,10 +902,9 @@ 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;