Re: [PATCH v6] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

[Luis Claudio R. Goncalves]

On Mon, Jul 28, 2025 at 10:14:41PM +0200, Oleg Nesterov wrote:
> On 07/07, Luis Claudio R. Goncalves wrote:
> >
> > Instead of adding more complex conditions to decide when to directly
> > call __put_task_struct() and when to defer the call, unconditionally
> > resort to the deferred call on PREEMPT_RT to simplify the code.
>                               ^^^^^^^^^^^^^
> Confused... with this patch put_task_struct() always uses the deferred
> call, regardless of PREEMPT_RT?
>
> Oleg.

You are correct.

I mistakenly sent the patch from v3, with the updated description. I had
been working on that patch in parallel for a future RFC and mixed them up
when updating the description for v6. The changes from v4 onward were
updates to the patch description only.

The fact that I had discussed this patch (the one submitted) with some
people and was asking for tests to assess robustness and dependability
may have further composed the confusion.

I posted v6 (with the wrong patch) a couple of hours before leaving for
a 2-week vacation. This is also why I didn't notice the wrong submission
before. That was an unfortunate mistake on my part, no bad intent.

Not sure how to proceed here, if I should resend the correct patch or a
follow-up fix like this:

======

From: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>
Subject: sched: restore the behavior of put_task_struct() for non-rt

Commit 8671bad873eb ("sched: Do not call __put_task_struct() on rt
if pi_blocked_on is set") changed the behavior of put_task_struct()
unconditionally, even when PREEMPT_RT was not enabled, in clear mismatch
with the commit description.

Restore the previous behavior of put_task_struct() for the PREEMPT_RT
disabled case.

Fixes: 8671bad873eb ("sched: Do not call __put_task_struct() on rt if pi_blocked_on is set")
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>
---
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index ea41795a352b..51678a541477 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -130,6 +133,16 @@ static inline void put_task_struct(struct task_struct *t)
 	if (!refcount_dec_and_test(&t->usage))
 		return;
 
+	/* In !RT, it is always safe to call __put_task_struct(). */
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP);
+
+		lock_map_acquire_try(&put_task_map);
+		__put_task_struct(t);
+		lock_map_release(&put_task_map);
+		return;
+	}
+
 	/*
 	 * Under PREEMPT_RT, we can't call __put_task_struct
 	 * in atomic context because it will indirectly
@@ -137,10 +150,6 @@ static inline void put_task_struct(struct task_struct *t)
 	 * current process has a mutex enqueued (blocked on
 	 * a PI chain).
 	 *
-	 * In !RT, it is always safe to call __put_task_struct().
-	 * Though, in order to simplify the code, resort to the
-	 * deferred call too.
-	 *
 	 * call_rcu() will schedule __put_task_struct_rcu_cb()
 	 * to be called in process context.
 	 *