sched_ext: Fix bypass depth leak on scx_enable() failure

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: 8c2090c ("sched_ext: Initialize in bypass mode")
Cc: stable@vger.kernel.org # v6.12+
Reported-by: Chris Mason <clm@meta.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/stable/286e6f7787a81239e1ce2989b52391ce%40kernel.org
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: htejun

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 kthread_work *work)
 	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_ops *ops, struct bpf_link *link)
 	 * 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_ops *ops, struct bpf_link *link)
 	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)) {