linux-kernel - Re: FAILED: Patch "posix-cpu-timers: Clear TICK_DEP_BIT_POSIX_TIMER on clone" failed to apply to v5.15-stable tree

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Message-ID: <ZyyYcdCYZfJfwDty@pavilion.home>
Date: Thu, 7 Nov 2024 11:37:37 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Sasha Levin <sashal@...nel.org>
Cc: stable@...r.kernel.org, bsegall@...gle.com,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: FAILED: Patch "posix-cpu-timers: Clear TICK_DEP_BIT_POSIX_TIMER
 on clone" failed to apply to v5.15-stable tree

Le Tue, Nov 05, 2024 at 09:11:36PM -0500, Sasha Levin a écrit :
> The patch below does not apply to the v5.15-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@...r.kernel.org>.
> 
> Thanks,
> Sasha

Please try this one instead:

>From 806ef66e95663c8fce77a7b9ffb8788b11105307 Mon Sep 17 00:00:00 2001
From: Benjamin Segall <bsegall@...gle.com>
Date: Fri, 25 Oct 2024 18:35:35 -0700
Subject: [PATCH] posix-cpu-timers: Clear TICK_DEP_BIT_POSIX_TIMER on clone

When cloning a new thread, its posix_cputimers are not inherited, and
are cleared by posix_cputimers_init(). However, this does not clear the
tick dependency it creates in tsk->tick_dep_mask, and the handler does
not reach the code to clear the dependency if there were no timers to
begin with.

Thus if a thread has a cputimer running before clone/fork, all
descendants will prevent nohz_full unless they create a cputimer of
their own.

Fix this by entirely clearing the tick_dep_mask in copy_process().
(There is currently no inherited state that needs a tick dependency)

Process-wide timers do not have this problem because fork does not copy
signal_struct as a baseline, it creates one from scratch.

Fixes: b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")
Signed-off-by: Ben Segall <bsegall@...gle.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/all/xm26o737bq8o.fsf@google.com
---
 include/linux/tick.h | 8 ++++++++
 kernel/fork.c        | 2 ++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 9459fef5b857..9701c571a5cf 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -252,12 +252,19 @@ static inline void tick_dep_set_task(struct task_struct *tsk,
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_set_task(tsk, bit);
 }
+
 static inline void tick_dep_clear_task(struct task_struct *tsk,
 				       enum tick_dep_bits bit)
 {
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_clear_task(tsk, bit);
 }
+
+static inline void tick_dep_init_task(struct task_struct *tsk)
+{
+	atomic_set(&tsk->tick_dep_mask, 0);
+}
+
 static inline void tick_dep_set_signal(struct task_struct *tsk,
 				       enum tick_dep_bits bit)
 {
@@ -291,6 +298,7 @@ static inline void tick_dep_set_task(struct task_struct *tsk,
 				     enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_task(struct task_struct *tsk,
 				       enum tick_dep_bits bit) { }
+static inline void tick_dep_init_task(struct task_struct *tsk) { }
 static inline void tick_dep_set_signal(struct task_struct *tsk,
 				       enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_signal(struct signal_struct *signal,
diff --git a/kernel/fork.c b/kernel/fork.c
index fec103ae640f..23ffcf8a859c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -97,6 +97,7 @@
 #include <linux/scs.h>
 #include <linux/io_uring.h>
 #include <linux/bpf.h>
+#include <linux/tick.h>
 
 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>
@@ -2126,6 +2127,7 @@ static __latent_entropy struct task_struct *copy_process(
 	acct_clear_integrals(p);
 
 	posix_cputimers_init(&p->posix_cputimers);
+	tick_dep_init_task(p);
 
 	p->io_context = NULL;
 	audit_set_context(p, NULL);
-- 
2.46.0