FAILED: patch "[PATCH] sched_ext: Disable preemption between scx_claim_exit() and" failed to apply to 6.18-stable tree

FAILED: patch "[PATCH] sched_ext: Disable preemption between scx_claim_exit() and" failed to apply to 6.18-stable tree

[gregkh]

The patch below does not apply to the 6.18-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@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 83236b2e43dba00bee5b82eb5758816b1a674f6a
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026031736-mystify-skating-036b@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 83236b2e43dba00bee5b82eb5758816b1a674f6a Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Tue, 24 Feb 2026 21:39:58 -1000
Subject: [PATCH] sched_ext: Disable preemption between scx_claim_exit() and
 kicking helper work

scx_claim_exit() atomically sets exit_kind, which prevents scx_error() from
triggering further error handling. After claiming exit, the caller must kick
the helper kthread work which initiates bypass mode and teardown.

If the calling task gets preempted between claiming exit and kicking the
helper work, and the BPF scheduler fails to schedule it back (since error
handling is now disabled), the helper work is never queued, bypass mode
never activates, tasks stop being dispatched, and the system wedges.

Disable preemption across scx_claim_exit() and the subsequent work kicking
in all callers - scx_disable() and scx_vexit(). Add
lockdep_assert_preemption_disabled() to scx_claim_exit() to enforce the
requirement.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Cc: stable@vger.kernel.org # v6.12+
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index c18e81e8ef51..9280381f8923 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4423,10 +4423,19 @@ static void scx_disable_workfn(struct kthread_work *work)
 	scx_bypass(false);
 }
 
+/*
+ * Claim the exit on @sch. The caller must ensure that the helper kthread work
+ * is kicked before the current task can be preempted. Once exit_kind is
+ * claimed, scx_error() can no longer trigger, so if the current task gets
+ * preempted and the BPF scheduler fails to schedule it back, the helper work
+ * will never be kicked and the whole system can wedge.
+ */
 static bool scx_claim_exit(struct scx_sched *sch, enum scx_exit_kind kind)
 {
 	int none = SCX_EXIT_NONE;
 
+	lockdep_assert_preemption_disabled();
+
 	if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind))
 		return false;
 
@@ -4449,6 +4458,7 @@ static void scx_disable(enum scx_exit_kind kind)
 	rcu_read_lock();
 	sch = rcu_dereference(scx_root);
 	if (sch) {
+		guard(preempt)();
 		scx_claim_exit(sch, kind);
 		kthread_queue_work(sch->helper, &sch->disable_work);
 	}
@@ -4771,6 +4781,8 @@ static bool scx_vexit(struct scx_sched *sch,
 {
 	struct scx_exit_info *ei = sch->exit_info;
 
+	guard(preempt)();
+
 	if (!scx_claim_exit(sch, kind))
 		return false;
 

[PATCH 6.18.y 1/2] sched_ext: Simplify breather mechanism with scx_aborting flag

[Sasha Levin]

From: Tejun Heo <tj@kernel.org>

[ Upstream commit a69040ed57f50156e5452474d25c79b9e62075d0 ]

The breather mechanism was introduced in 62dcbab8b0ef ("sched_ext: Avoid
live-locking bypass mode switching") and e32c260195e6 ("sched_ext: Enable the
ops breather and eject BPF scheduler on softlockup") to prevent live-locks by
injecting delays when CPUs are trapped in dispatch paths.

Currently, it uses scx_breather_depth (atomic_t) and scx_in_softlockup
(unsigned long) with separate increment/decrement and cleanup operations. The
breather is only activated when aborting, so tie it directly to the exit
mechanism. Replace both variables with scx_aborting flag set when exit is
claimed and cleared after bypass is enabled. Introduce scx_claim_exit() to
consolidate exit_kind claiming and breather enablement. This eliminates
scx_clear_softlockup() and simplifies scx_softlockup() and scx_bypass().

The breather mechanism will be replaced by a different abort mechanism in a
future patch. This simplification prepares for that change.

Reviewed-by: Dan Schatzberg <schatzberg.dan@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Stable-dep-of: 83236b2e43db ("sched_ext: Disable preemption between scx_claim_exit() and kicking helper work")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/ext.c | 54 +++++++++++++++++++++-------------------------
 1 file changed, 25 insertions(+), 29 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 2ff7034841c7c..a10344f00411d 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -33,9 +33,8 @@ static DEFINE_MUTEX(scx_enable_mutex);
 DEFINE_STATIC_KEY_FALSE(__scx_enabled);
 DEFINE_STATIC_PERCPU_RWSEM(scx_fork_rwsem);
 static atomic_t scx_enable_state_var = ATOMIC_INIT(SCX_DISABLED);
-static unsigned long scx_in_softlockup;
-static atomic_t scx_breather_depth = ATOMIC_INIT(0);
 static int scx_bypass_depth;
+static bool scx_aborting;
 static bool scx_init_task_enabled;
 static bool scx_switching_all;
 DEFINE_STATIC_KEY_FALSE(__scx_switched_all);
@@ -1792,7 +1791,7 @@ static void scx_breather(struct rq *rq)
 
 	lockdep_assert_rq_held(rq);
 
-	if (likely(!atomic_read(&scx_breather_depth)))
+	if (likely(!READ_ONCE(scx_aborting)))
 		return;
 
 	raw_spin_rq_unlock(rq);
@@ -1801,9 +1800,9 @@ static void scx_breather(struct rq *rq)
 
 	do {
 		int cnt = 1024;
-		while (atomic_read(&scx_breather_depth) && --cnt)
+		while (READ_ONCE(scx_aborting) && --cnt)
 			cpu_relax();
-	} while (atomic_read(&scx_breather_depth) &&
+	} while (READ_ONCE(scx_aborting) &&
 		 time_before64(ktime_get_ns(), until));
 
 	raw_spin_rq_lock(rq);
@@ -3720,30 +3719,14 @@ void scx_softlockup(u32 dur_s)
 		goto out_unlock;
 	}
 
-	/* allow only one instance, cleared at the end of scx_bypass() */
-	if (test_and_set_bit(0, &scx_in_softlockup))
-		goto out_unlock;
-
 	printk_deferred(KERN_ERR "sched_ext: Soft lockup - CPU%d stuck for %us, disabling \"%s\"\n",
 			smp_processor_id(), dur_s, scx_root->ops.name);
 
-	/*
-	 * Some CPUs may be trapped in the dispatch paths. Enable breather
-	 * immediately; otherwise, we might even be able to get to scx_bypass().
-	 */
-	atomic_inc(&scx_breather_depth);
-
 	scx_error(sch, "soft lockup - CPU#%d stuck for %us", smp_processor_id(), dur_s);
 out_unlock:
 	rcu_read_unlock();
 }
 
-static void scx_clear_softlockup(void)
-{
-	if (test_and_clear_bit(0, &scx_in_softlockup))
-		atomic_dec(&scx_breather_depth);
-}
-
 /**
  * scx_bypass - [Un]bypass scx_ops and guarantee forward progress
  * @bypass: true for bypass, false for unbypass
@@ -3804,8 +3787,6 @@ static void scx_bypass(bool bypass)
 				      ktime_get_ns() - bypass_timestamp);
 	}
 
-	atomic_inc(&scx_breather_depth);
-
 	/*
 	 * No task property is changing. We just need to make sure all currently
 	 * queued tasks are re-queued according to the new scx_rq_bypassing()
@@ -3862,10 +3843,8 @@ static void scx_bypass(bool bypass)
 		raw_spin_rq_unlock(rq);
 	}
 
-	atomic_dec(&scx_breather_depth);
 unlock:
 	raw_spin_unlock_irqrestore(&bypass_lock, flags);
-	scx_clear_softlockup();
 }
 
 static void free_exit_info(struct scx_exit_info *ei)
@@ -3960,6 +3939,7 @@ static void scx_disable_workfn(struct kthread_work *work)
 
 	/* guarantee forward progress by bypassing scx_ops */
 	scx_bypass(true);
+	WRITE_ONCE(scx_aborting, false);
 
 	switch (scx_set_enable_state(SCX_DISABLING)) {
 	case SCX_DISABLING:
@@ -4088,9 +4068,24 @@ static void scx_disable_workfn(struct kthread_work *work)
 	scx_bypass(false);
 }
 
-static void scx_disable(enum scx_exit_kind kind)
+static bool scx_claim_exit(struct scx_sched *sch, enum scx_exit_kind kind)
 {
 	int none = SCX_EXIT_NONE;
+
+	if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind))
+		return false;
+
+	/*
+	 * Some CPUs may be trapped in the dispatch paths. Enable breather
+	 * immediately; otherwise, we might not even be able to get to
+	 * scx_bypass().
+	 */
+	WRITE_ONCE(scx_aborting, true);
+	return true;
+}
+
+static void scx_disable(enum scx_exit_kind kind)
+{
 	struct scx_sched *sch;
 
 	if (WARN_ON_ONCE(kind == SCX_EXIT_NONE || kind == SCX_EXIT_DONE))
@@ -4099,7 +4094,7 @@ static void scx_disable(enum scx_exit_kind kind)
 	rcu_read_lock();
 	sch = rcu_dereference(scx_root);
 	if (sch) {
-		atomic_try_cmpxchg(&sch->exit_kind, &none, kind);
+		scx_claim_exit(sch, kind);
 		kthread_queue_work(sch->helper, &sch->disable_work);
 	}
 	rcu_read_unlock();
@@ -4420,9 +4415,8 @@ static void scx_vexit(struct scx_sched *sch,
 		      const char *fmt, va_list args)
 {
 	struct scx_exit_info *ei = sch->exit_info;
-	int none = SCX_EXIT_NONE;
 
-	if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind))
+	if (!scx_claim_exit(sch, kind))
 		return;
 
 	ei->exit_code = exit_code;
@@ -4637,6 +4631,8 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 	 */
 	WARN_ON_ONCE(scx_set_enable_state(SCX_ENABLING) != SCX_DISABLED);
 	WARN_ON_ONCE(scx_root);
+	if (WARN_ON_ONCE(READ_ONCE(scx_aborting)))
+		WRITE_ONCE(scx_aborting, false);
 
 	atomic_long_set(&scx_nr_rejected, 0);
 
-- 
2.51.0

[PATCH 6.18.y 2/2] sched_ext: Disable preemption between scx_claim_exit() and kicking helper work

[Sasha Levin]

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 83236b2e43dba00bee5b82eb5758816b1a674f6a ]

scx_claim_exit() atomically sets exit_kind, which prevents scx_error() from
triggering further error handling. After claiming exit, the caller must kick
the helper kthread work which initiates bypass mode and teardown.

If the calling task gets preempted between claiming exit and kicking the
helper work, and the BPF scheduler fails to schedule it back (since error
handling is now disabled), the helper work is never queued, bypass mode
never activates, tasks stop being dispatched, and the system wedges.

Disable preemption across scx_claim_exit() and the subsequent work kicking
in all callers - scx_disable() and scx_vexit(). Add
lockdep_assert_preemption_disabled() to scx_claim_exit() to enforce the
requirement.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Cc: stable@vger.kernel.org # v6.12+
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/ext.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index a10344f00411d..e565e622cd5ca 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4068,10 +4068,19 @@ static void scx_disable_workfn(struct kthread_work *work)
 	scx_bypass(false);
 }
 
+/*
+ * Claim the exit on @sch. The caller must ensure that the helper kthread work
+ * is kicked before the current task can be preempted. Once exit_kind is
+ * claimed, scx_error() can no longer trigger, so if the current task gets
+ * preempted and the BPF scheduler fails to schedule it back, the helper work
+ * will never be kicked and the whole system can wedge.
+ */
 static bool scx_claim_exit(struct scx_sched *sch, enum scx_exit_kind kind)
 {
 	int none = SCX_EXIT_NONE;
 
+	lockdep_assert_preemption_disabled();
+
 	if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind))
 		return false;
 
@@ -4094,6 +4103,7 @@ static void scx_disable(enum scx_exit_kind kind)
 	rcu_read_lock();
 	sch = rcu_dereference(scx_root);
 	if (sch) {
+		guard(preempt)();
 		scx_claim_exit(sch, kind);
 		kthread_queue_work(sch->helper, &sch->disable_work);
 	}
@@ -4416,6 +4426,8 @@ static void scx_vexit(struct scx_sched *sch,
 {
 	struct scx_exit_info *ei = sch->exit_info;
 
+	guard(preempt)();
+
 	if (!scx_claim_exit(sch, kind))
 		return;
 
-- 
2.51.0