sched_ext: Fix bypass depth leak on scx_enable() failure

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

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 [email protected] # v6.12+

Reported-by Chris Mason <[email protected]>

Signed-off-by Tejun Heo <[email protected]>

---
 kernel/sched/ext.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

kernel/sched/ext.c +14 -1

@@ -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