[RFC PATCH 07/86] Revert "livepatch,sched: Add livepatch task switching to cond_resched()"

[Ankur Arora <ankur.a.arora@oracle.com>]

This reverts commit e3ff7c609f39671d1aaff4fb4a8594e14f3e03f8.

Note that removing this commit reintroduces "live patches failing to
complete within a reasonable amount of time due to CPU-bound kthreads."

Unfortunately this fix depends quite critically on PREEMPT_DYNAMIC and
existence of cond_resched() so this will need an alternate fix.

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 include/linux/livepatch.h       |   1 -
 include/linux/livepatch_sched.h |  29 ---------
 include/linux/sched.h           |  20 ++----
 kernel/livepatch/core.c         |   1 -
 kernel/livepatch/transition.c   | 107 +++++---------------------------
 kernel/sched/core.c             |  64 +++----------------
 6 files changed, 28 insertions(+), 194 deletions(-)
 delete mode 100644 include/linux/livepatch_sched.h

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 9b9b38e89563..293e29960c6e 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -13,7 +13,6 @@
 #include <linux/ftrace.h>
 #include <linux/completion.h>
 #include <linux/list.h>
-#include <linux/livepatch_sched.h>
 
 #if IS_ENABLED(CONFIG_LIVEPATCH)
 
diff --git a/include/linux/livepatch_sched.h b/include/linux/livepatch_sched.h
deleted file mode 100644
index 013794fb5da0..000000000000
--- a/include/linux/livepatch_sched.h
+++ /dev/null
@@ -1,29 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0-or-later */
-#ifndef _LINUX_LIVEPATCH_SCHED_H_
-#define _LINUX_LIVEPATCH_SCHED_H_
-
-#include <linux/jump_label.h>
-#include <linux/static_call_types.h>
-
-#ifdef CONFIG_LIVEPATCH
-
-void __klp_sched_try_switch(void);
-
-#if !defined(CONFIG_PREEMPT_DYNAMIC) || !defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
-
-DECLARE_STATIC_KEY_FALSE(klp_sched_try_switch_key);
-
-static __always_inline void klp_sched_try_switch(void)
-{
-	if (static_branch_unlikely(&klp_sched_try_switch_key))
-		__klp_sched_try_switch();
-}
-
-#endif /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
-
-#else /* !CONFIG_LIVEPATCH */
-static inline void klp_sched_try_switch(void) {}
-static inline void __klp_sched_try_switch(void) {}
-#endif /* CONFIG_LIVEPATCH */
-
-#endif /* _LINUX_LIVEPATCH_SCHED_H_ */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5bdf80136e42..c5b0ef1ecfe4 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -36,7 +36,6 @@
 #include <linux/seqlock.h>
 #include <linux/kcsan.h>
 #include <linux/rv.h>
-#include <linux/livepatch_sched.h>
 #include <asm/kmap_size.h>
 
 /* task_struct member predeclarations (sorted alphabetically): */
@@ -2087,9 +2086,6 @@ extern int __cond_resched(void);
 
 #if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
 
-void sched_dynamic_klp_enable(void);
-void sched_dynamic_klp_disable(void);
-
 DECLARE_STATIC_CALL(cond_resched, __cond_resched);
 
 static __always_inline int _cond_resched(void)
@@ -2098,7 +2094,6 @@ static __always_inline int _cond_resched(void)
 }
 
 #elif defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
-
 extern int dynamic_cond_resched(void);
 
 static __always_inline int _cond_resched(void)
@@ -2106,25 +2101,20 @@ static __always_inline int _cond_resched(void)
 	return dynamic_cond_resched();
 }
 
-#else /* !CONFIG_PREEMPTION */
+#else
 
 static inline int _cond_resched(void)
 {
-	klp_sched_try_switch();
 	return __cond_resched();
 }
 
-#endif /* PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
+#endif /* CONFIG_PREEMPT_DYNAMIC */
 
-#else /* CONFIG_PREEMPTION && !CONFIG_PREEMPT_DYNAMIC */
+#else
 
-static inline int _cond_resched(void)
-{
-	klp_sched_try_switch();
-	return 0;
-}
+static inline int _cond_resched(void) { return 0; }
 
-#endif /* !CONFIG_PREEMPTION || CONFIG_PREEMPT_DYNAMIC */
+#endif /* !defined(CONFIG_PREEMPTION) || defined(CONFIG_PREEMPT_DYNAMIC) */
 
 #define cond_resched() ({			\
 	__might_resched(__FILE__, __LINE__, 0);	\
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 61328328c474..fc851455740c 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -33,7 +33,6 @@
  *
  * - klp_ftrace_handler()
  * - klp_update_patch_state()
- * - __klp_sched_try_switch()
  */
 DEFINE_MUTEX(klp_mutex);
 
diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index e54c3d60a904..70bc38f27af7 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -9,7 +9,6 @@
 
 #include <linux/cpu.h>
 #include <linux/stacktrace.h>
-#include <linux/static_call.h>
 #include "core.h"
 #include "patch.h"
 #include "transition.h"
@@ -27,25 +26,6 @@ static int klp_target_state = KLP_UNDEFINED;
 
 static unsigned int klp_signals_cnt;
 
-/*
- * When a livepatch is in progress, enable klp stack checking in
- * cond_resched().  This helps CPU-bound kthreads get patched.
- */
-#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
-
-#define klp_cond_resched_enable() sched_dynamic_klp_enable()
-#define klp_cond_resched_disable() sched_dynamic_klp_disable()
-
-#else /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
-
-DEFINE_STATIC_KEY_FALSE(klp_sched_try_switch_key);
-EXPORT_SYMBOL(klp_sched_try_switch_key);
-
-#define klp_cond_resched_enable() static_branch_enable(&klp_sched_try_switch_key)
-#define klp_cond_resched_disable() static_branch_disable(&klp_sched_try_switch_key)
-
-#endif /* CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
-
 /*
  * This work can be performed periodically to finish patching or unpatching any
  * "straggler" tasks which failed to transition in the first attempt.
@@ -194,8 +174,8 @@ void klp_update_patch_state(struct task_struct *task)
 	 * barrier (smp_rmb) for two cases:
 	 *
 	 * 1) Enforce the order of the TIF_PATCH_PENDING read and the
-	 *    klp_target_state read.  The corresponding write barriers are in
-	 *    klp_init_transition() and klp_reverse_transition().
+	 *    klp_target_state read.  The corresponding write barrier is in
+	 *    klp_init_transition().
 	 *
 	 * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
 	 *    of func->transition, if klp_ftrace_handler() is called later on
@@ -363,44 +343,6 @@ static bool klp_try_switch_task(struct task_struct *task)
 	return !ret;
 }
 
-void __klp_sched_try_switch(void)
-{
-	if (likely(!klp_patch_pending(current)))
-		return;
-
-	/*
-	 * This function is called from cond_resched() which is called in many
-	 * places throughout the kernel.  Using the klp_mutex here might
-	 * deadlock.
-	 *
-	 * Instead, disable preemption to prevent racing with other callers of
-	 * klp_try_switch_task().  Thanks to task_call_func() they won't be
-	 * able to switch this task while it's running.
-	 */
-	preempt_disable();
-
-	/*
-	 * Make sure current didn't get patched between the above check and
-	 * preempt_disable().
-	 */
-	if (unlikely(!klp_patch_pending(current)))
-		goto out;
-
-	/*
-	 * Enforce the order of the TIF_PATCH_PENDING read above and the
-	 * klp_target_state read in klp_try_switch_task().  The corresponding
-	 * write barriers are in klp_init_transition() and
-	 * klp_reverse_transition().
-	 */
-	smp_rmb();
-
-	klp_try_switch_task(current);
-
-out:
-	preempt_enable();
-}
-EXPORT_SYMBOL(__klp_sched_try_switch);
-
 /*
  * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
  * Kthreads with TIF_PATCH_PENDING set are woken up.
@@ -507,8 +449,7 @@ void klp_try_complete_transition(void)
 		return;
 	}
 
-	/* Done!  Now cleanup the data structures. */
-	klp_cond_resched_disable();
+	/* we're done, now cleanup the data structures */
 	patch = klp_transition_patch;
 	klp_complete_transition();
 
@@ -560,8 +501,6 @@ void klp_start_transition(void)
 			set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 	}
 
-	klp_cond_resched_enable();
-
 	klp_signals_cnt = 0;
 }
 
@@ -617,9 +556,8 @@ void klp_init_transition(struct klp_patch *patch, int state)
 	 * see a func in transition with a task->patch_state of KLP_UNDEFINED.
 	 *
 	 * Also enforce the order of the klp_target_state write and future
-	 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() and
-	 * __klp_sched_try_switch() don't set a task->patch_state to
-	 * KLP_UNDEFINED.
+	 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
+	 * set a task->patch_state to KLP_UNDEFINED.
 	 */
 	smp_wmb();
 
@@ -655,10 +593,14 @@ void klp_reverse_transition(void)
 		 klp_target_state == KLP_PATCHED ? "patching to unpatching" :
 						   "unpatching to patching");
 
+	klp_transition_patch->enabled = !klp_transition_patch->enabled;
+
+	klp_target_state = !klp_target_state;
+
 	/*
 	 * Clear all TIF_PATCH_PENDING flags to prevent races caused by
-	 * klp_update_patch_state() or __klp_sched_try_switch() running in
-	 * parallel with the reverse transition.
+	 * klp_update_patch_state() running in parallel with
+	 * klp_start_transition().
 	 */
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, task)
@@ -668,28 +610,9 @@ void klp_reverse_transition(void)
 	for_each_possible_cpu(cpu)
 		clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
 
-	/*
-	 * Make sure all existing invocations of klp_update_patch_state() and
-	 * __klp_sched_try_switch() see the cleared TIF_PATCH_PENDING before
-	 * starting the reverse transition.
-	 */
+	/* Let any remaining calls to klp_update_patch_state() complete */
 	klp_synchronize_transition();
 
-	/*
-	 * All patching has stopped, now re-initialize the global variables to
-	 * prepare for the reverse transition.
-	 */
-	klp_transition_patch->enabled = !klp_transition_patch->enabled;
-	klp_target_state = !klp_target_state;
-
-	/*
-	 * Enforce the order of the klp_target_state write and the
-	 * TIF_PATCH_PENDING writes in klp_start_transition() to ensure
-	 * klp_update_patch_state() and __klp_sched_try_switch() don't set
-	 * task->patch_state to the wrong value.
-	 */
-	smp_wmb();
-
 	klp_start_transition();
 }
 
@@ -703,9 +626,9 @@ void klp_copy_process(struct task_struct *child)
 	 * the task flag up to date with the parent here.
 	 *
 	 * The operation is serialized against all klp_*_transition()
-	 * operations by the tasklist_lock. The only exceptions are
-	 * klp_update_patch_state(current) and __klp_sched_try_switch(), but we
-	 * cannot race with them because we are current.
+	 * operations by the tasklist_lock. The only exception is
+	 * klp_update_patch_state(current), but we cannot race with
+	 * that because we are current.
 	 */
 	if (test_tsk_thread_flag(current, TIF_PATCH_PENDING))
 		set_tsk_thread_flag(child, TIF_PATCH_PENDING);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0e8764d63041..b43fda3c5733 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8597,7 +8597,6 @@ EXPORT_STATIC_CALL_TRAMP(might_resched);
 static DEFINE_STATIC_KEY_FALSE(sk_dynamic_cond_resched);
 int __sched dynamic_cond_resched(void)
 {
-	klp_sched_try_switch();
 	if (!static_branch_unlikely(&sk_dynamic_cond_resched))
 		return 0;
 	return __cond_resched();
@@ -8746,17 +8745,13 @@ int sched_dynamic_mode(const char *str)
 #error "Unsupported PREEMPT_DYNAMIC mechanism"
 #endif
 
-DEFINE_MUTEX(sched_dynamic_mutex);
-static bool klp_override;
-
-static void __sched_dynamic_update(int mode)
+void sched_dynamic_update(int mode)
 {
 	/*
 	 * Avoid {NONE,VOLUNTARY} -> FULL transitions from ever ending up in
 	 * the ZERO state, which is invalid.
 	 */
-	if (!klp_override)
-		preempt_dynamic_enable(cond_resched);
+	preempt_dynamic_enable(cond_resched);
 	preempt_dynamic_enable(might_resched);
 	preempt_dynamic_enable(preempt_schedule);
 	preempt_dynamic_enable(preempt_schedule_notrace);
@@ -8764,79 +8759,36 @@ static void __sched_dynamic_update(int mode)
 
 	switch (mode) {
 	case preempt_dynamic_none:
-		if (!klp_override)
-			preempt_dynamic_enable(cond_resched);
+		preempt_dynamic_enable(cond_resched);
 		preempt_dynamic_disable(might_resched);
 		preempt_dynamic_disable(preempt_schedule);
 		preempt_dynamic_disable(preempt_schedule_notrace);
 		preempt_dynamic_disable(irqentry_exit_cond_resched);
-		if (mode != preempt_dynamic_mode)
-			pr_info("Dynamic Preempt: none\n");
+		pr_info("Dynamic Preempt: none\n");
 		break;
 
 	case preempt_dynamic_voluntary:
-		if (!klp_override)
-			preempt_dynamic_enable(cond_resched);
+		preempt_dynamic_enable(cond_resched);
 		preempt_dynamic_enable(might_resched);
 		preempt_dynamic_disable(preempt_schedule);
 		preempt_dynamic_disable(preempt_schedule_notrace);
 		preempt_dynamic_disable(irqentry_exit_cond_resched);
-		if (mode != preempt_dynamic_mode)
-			pr_info("Dynamic Preempt: voluntary\n");
+		pr_info("Dynamic Preempt: voluntary\n");
 		break;
 
 	case preempt_dynamic_full:
-		if (!klp_override)
-			preempt_dynamic_disable(cond_resched);
+		preempt_dynamic_disable(cond_resched);
 		preempt_dynamic_disable(might_resched);
 		preempt_dynamic_enable(preempt_schedule);
 		preempt_dynamic_enable(preempt_schedule_notrace);
 		preempt_dynamic_enable(irqentry_exit_cond_resched);
-		if (mode != preempt_dynamic_mode)
-			pr_info("Dynamic Preempt: full\n");
+		pr_info("Dynamic Preempt: full\n");
 		break;
 	}
 
 	preempt_dynamic_mode = mode;
 }
 
-void sched_dynamic_update(int mode)
-{
-	mutex_lock(&sched_dynamic_mutex);
-	__sched_dynamic_update(mode);
-	mutex_unlock(&sched_dynamic_mutex);
-}
-
-#ifdef CONFIG_HAVE_PREEMPT_DYNAMIC_CALL
-
-static int klp_cond_resched(void)
-{
-	__klp_sched_try_switch();
-	return __cond_resched();
-}
-
-void sched_dynamic_klp_enable(void)
-{
-	mutex_lock(&sched_dynamic_mutex);
-
-	klp_override = true;
-	static_call_update(cond_resched, klp_cond_resched);
-
-	mutex_unlock(&sched_dynamic_mutex);
-}
-
-void sched_dynamic_klp_disable(void)
-{
-	mutex_lock(&sched_dynamic_mutex);
-
-	klp_override = false;
-	__sched_dynamic_update(preempt_dynamic_mode);
-
-	mutex_unlock(&sched_dynamic_mutex);
-}
-
-#endif /* CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
-
 static int __init setup_preempt_mode(char *str)
 {
 	int mode = sched_dynamic_mode(str);
-- 
2.31.1