[PATCH sched_ext/for-6.19-fixes] sched_ext: Fix bypass depth leak on scx_enable() failure

[PATCH sched_ext/for-6.19-fixes] sched_ext: Fix bypass depth leak on scx_enable() failure

[Tejun Heo]

scx_enable() calls scx_bypass(true) to initialize in bypass mode and then
scx_bypass(false) on success to exit. If scx_enable() fails during task
initialization - e.g. scx_cgroup_init() or scx_init_task() returns an error -
it jumps to err_disable while bypass is still active. scx_disable_workfn()
then calls scx_bypass(true/false) for its own bypass, leaving the bypass depth
at 1 instead of 0. This causes the system to remain permanently in bypass mode
after a failed scx_enable().

Failures after task initialization is complete - e.g. scx_tryset_enable_state()
at the end - already call scx_bypass(false) before reaching the error path and
are not affected. This only affects a subset of failure modes.

Fix it by tracking whether scx_enable() called scx_bypass(true) in a bool and
having scx_disable_workfn() call an extra scx_bypass(false) to clear it. This
is a temporary measure as the bypass depth will be moved into the sched
instance, which will make this tracking unnecessary.

Fixes: 8c2090c504e9 ("sched_ext: Initialize in bypass mode")
Cc: stable@vger.kernel.org # v6.12+
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -41,6 +41,13 @@ static bool scx_init_task_enabled;
 static bool scx_switching_all;
 DEFINE_STATIC_KEY_FALSE(__scx_switched_all);

+/*
+ * Tracks whether scx_enable() called scx_bypass(true). Used to balance bypass
+ * depth on enable failure. Will be removed when bypass depth is moved into the
+ * sched instance.
+ */
+static bool scx_bypassed_for_enable;
+
 static atomic_long_t scx_nr_rejected = ATOMIC_LONG_INIT(0);
 static atomic_long_t scx_hotplug_seq = ATOMIC_LONG_INIT(0);

@@ -4318,6 +4325,11 @@ static void scx_disable_workfn(struct kt
 	scx_dsp_max_batch = 0;
 	free_kick_syncs();

+	if (scx_bypassed_for_enable) {
+		scx_bypassed_for_enable = false;
+		scx_bypass(false);
+	}
+
 	mutex_unlock(&scx_enable_mutex);

 	WARN_ON_ONCE(scx_set_enable_state(SCX_DISABLED) != SCX_DISABLING);
@@ -4970,6 +4982,7 @@ static int scx_enable(struct sched_ext_o
 	 * Init in bypass mode to guarantee forward progress.
 	 */
 	scx_bypass(true);
+	scx_bypassed_for_enable = true;

 	for (i = SCX_OPI_NORMAL_BEGIN; i < SCX_OPI_NORMAL_END; i++)
 		if (((void (**)(void))ops)[i])
@@ -5067,6 +5080,7 @@ static int scx_enable(struct sched_ext_o
 	scx_task_iter_stop(&sti);
 	percpu_up_write(&scx_fork_rwsem);

+	scx_bypassed_for_enable = false;
 	scx_bypass(false);

 	if (!scx_tryset_enable_state(SCX_ENABLED, SCX_ENABLING)) {
--
tejun

Re: [PATCH sched_ext/for-6.19-fixes] sched_ext: Fix bypass depth leak on scx_enable() failure

[kernel test robot]

Hi,

Thanks for your patch.

FYI: kernel test robot notices the stable kernel rule is not satisfied.

The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#option-3

Rule: The upstream commit ID must be specified with a separate line above the commit text.
Subject: [PATCH sched_ext/for-6.19-fixes] sched_ext: Fix bypass depth leak on scx_enable() failure
Link: https://lore.kernel.org/stable/286e6f7787a81239e1ce2989b52391ce%40kernel.org

Please ignore this mail if the patch is not relevant for upstream.

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH sched_ext/for-6.19-fixes] sched_ext: Fix bypass depth leak on scx_enable() failure

[Greg KH]

On Wed, Dec 10, 2025 at 05:13:38AM +0800, kernel test robot wrote:
> Hi,
> 
> Thanks for your patch.
> 
> FYI: kernel test robot notices the stable kernel rule is not satisfied.
> 
> The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#option-3

Something went wrong, this seemed correct :(

Re: [PATCH sched_ext/for-6.19-fixes] sched_ext: Fix bypass depth leak on scx_enable() failure

[Emil Tsalapatis]

On Tue Dec 9, 2025 at 4:04 PM EST, Tejun Heo wrote:
> scx_enable() calls scx_bypass(true) to initialize in bypass mode and then
> scx_bypass(false) on success to exit. If scx_enable() fails during task
> initialization - e.g. scx_cgroup_init() or scx_init_task() returns an error -
> it jumps to err_disable while bypass is still active. scx_disable_workfn()
> then calls scx_bypass(true/false) for its own bypass, leaving the bypass depth
> at 1 instead of 0. This causes the system to remain permanently in bypass mode
> after a failed scx_enable().
>
> Failures after task initialization is complete - e.g. scx_tryset_enable_state()
> at the end - already call scx_bypass(false) before reaching the error path and
> are not affected. This only affects a subset of failure modes.
>
> Fix it by tracking whether scx_enable() called scx_bypass(true) in a bool and
> having scx_disable_workfn() call an extra scx_bypass(false) to clear it. This
> is a temporary measure as the bypass depth will be moved into the sched
> instance, which will make this tracking unnecessary.
>
> Fixes: 8c2090c504e9 ("sched_ext: Initialize in bypass mode")
> Cc: stable@vger.kernel.org # v6.12+
> Reported-by: Chris Mason <clm@meta.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>


> ---
>  kernel/sched/ext.c |   14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -41,6 +41,13 @@ static bool scx_init_task_enabled;
>  static bool scx_switching_all;
>  DEFINE_STATIC_KEY_FALSE(__scx_switched_all);
>
> +/*
> + * Tracks whether scx_enable() called scx_bypass(true). Used to balance bypass
> + * depth on enable failure. Will be removed when bypass depth is moved into the
> + * sched instance.
> + */
> +static bool scx_bypassed_for_enable;
> +
>  static atomic_long_t scx_nr_rejected = ATOMIC_LONG_INIT(0);
>  static atomic_long_t scx_hotplug_seq = ATOMIC_LONG_INIT(0);
>
> @@ -4318,6 +4325,11 @@ static void scx_disable_workfn(struct kt
>  	scx_dsp_max_batch = 0;
>  	free_kick_syncs();
>
> +	if (scx_bypassed_for_enable) {
> +		scx_bypassed_for_enable = false;
> +		scx_bypass(false);
> +	}
> +
>  	mutex_unlock(&scx_enable_mutex);
>
>  	WARN_ON_ONCE(scx_set_enable_state(SCX_DISABLED) != SCX_DISABLING);
> @@ -4970,6 +4982,7 @@ static int scx_enable(struct sched_ext_o
>  	 * Init in bypass mode to guarantee forward progress.
>  	 */
>  	scx_bypass(true);
> +	scx_bypassed_for_enable = true;
>
>  	for (i = SCX_OPI_NORMAL_BEGIN; i < SCX_OPI_NORMAL_END; i++)
>  		if (((void (**)(void))ops)[i])
> @@ -5067,6 +5080,7 @@ static int scx_enable(struct sched_ext_o
>  	scx_task_iter_stop(&sti);
>  	percpu_up_write(&scx_fork_rwsem);
>
> +	scx_bypassed_for_enable = false;
>  	scx_bypass(false);
>
>  	if (!scx_tryset_enable_state(SCX_ENABLED, SCX_ENABLING)) {
> --
> tejun

Re: [PATCH sched_ext/for-6.19-fixes] sched_ext: Fix bypass depth leak on scx_enable() failure

[Tejun Heo]

Applied to sched_ext/for-6.19-fixes.

Thanks.
--
tejun