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Subject[tip: sched/urgent] tasks: Add a count of task RCU users
The following commit has been merged into the sched/urgent branch of tip:

Commit-ID: 3fbd7ee285b2bbc6eebd15a3c8786d9776a402a8
Gitweb: https://git.kernel.org/tip/3fbd7ee285b2bbc6eebd15a3c8786d9776a402a8
Author: Eric W. Biederman <ebiederm@xmission.com>
AuthorDate: Sat, 14 Sep 2019 07:33:34 -05:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 25 Sep 2019 17:42:29 +02:00

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>
---
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 e2e9196..8e43e54 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1147,7 +1147,10 @@ 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 3d90ed8..153a683 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -120,6 +120,7 @@ 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 22ab6a4..3bcaec2 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 @@ repeat:

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 1d1cd06..7eefe33 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;
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