[PATCH v7 2/5] lazy tlb: introduce lazy tlb mm refcount helper functions

[Nicholas Piggin <npiggin@gmail.com>]

Add explicit _lazy_tlb annotated functions for lazy tlb mm refcounting.
This makes the lazy tlb mm references more obvious, and allows the
refcounting scheme to be modified in later changes. There is no
functional change with this patch.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/arm/mach-rpc/ecard.c            |  2 +-
 arch/powerpc/kernel/smp.c            |  2 +-
 arch/powerpc/mm/book3s64/radix_tlb.c |  4 ++--
 fs/exec.c                            |  2 +-
 include/linux/sched/mm.h             | 16 ++++++++++++++++
 kernel/cpu.c                         |  2 +-
 kernel/exit.c                        |  2 +-
 kernel/kthread.c                     | 12 ++++++++++--
 kernel/sched/core.c                  | 15 ++++++++-------
 9 files changed, 41 insertions(+), 16 deletions(-)

diff --git a/arch/arm/mach-rpc/ecard.c b/arch/arm/mach-rpc/ecard.c
index 53813f9464a2..c30df1097c52 100644
--- a/arch/arm/mach-rpc/ecard.c
+++ b/arch/arm/mach-rpc/ecard.c
@@ -253,7 +253,7 @@ static int ecard_init_mm(void)
 	current->mm = mm;
 	current->active_mm = mm;
 	activate_mm(active_mm, mm);
-	mmdrop(active_mm);
+	mmdrop_lazy_tlb(active_mm);
 	ecard_init_pgtables(mm);
 	return 0;
 }
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 6b90f10a6c81..7db6b3faea65 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1611,7 +1611,7 @@ void start_secondary(void *unused)
 	if (IS_ENABLED(CONFIG_PPC32))
 		setup_kup();
 
-	mmgrab(&init_mm);
+	mmgrab_lazy_tlb(&init_mm);
 	current->active_mm = &init_mm;
 
 	smp_store_cpu_info(cpu);
diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
index 4e29b619578c..282359ab525b 100644
--- a/arch/powerpc/mm/book3s64/radix_tlb.c
+++ b/arch/powerpc/mm/book3s64/radix_tlb.c
@@ -794,10 +794,10 @@ void exit_lazy_flush_tlb(struct mm_struct *mm, bool always_flush)
 	if (current->active_mm == mm) {
 		WARN_ON_ONCE(current->mm != NULL);
 		/* Is a kernel thread and is using mm as the lazy tlb */
-		mmgrab(&init_mm);
+		mmgrab_lazy_tlb(&init_mm);
 		current->active_mm = &init_mm;
 		switch_mm_irqs_off(mm, &init_mm, current);
-		mmdrop(mm);
+		mmdrop_lazy_tlb(mm);
 	}
 
 	/*
diff --git a/fs/exec.c b/fs/exec.c
index ab913243a367..1a32a88db173 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1033,7 +1033,7 @@ static int exec_mmap(struct mm_struct *mm)
 		mmput(old_mm);
 		return 0;
 	}
-	mmdrop(active_mm);
+	mmdrop_lazy_tlb(active_mm);
 	return 0;
 }
 
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 2a243616f222..5376caf6fcf3 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -79,6 +79,22 @@ static inline void mmdrop_sched(struct mm_struct *mm)
 }
 #endif
 
+/* Helpers for lazy TLB mm refcounting */
+static inline void mmgrab_lazy_tlb(struct mm_struct *mm)
+{
+	mmgrab(mm);
+}
+
+static inline void mmdrop_lazy_tlb(struct mm_struct *mm)
+{
+	mmdrop(mm);
+}
+
+static inline void mmdrop_lazy_tlb_sched(struct mm_struct *mm)
+{
+	mmdrop_sched(mm);
+}
+
 /**
  * mmget() - Pin the address space associated with a &struct mm_struct.
  * @mm: The address space to pin.
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6c0a92ca6bb5..189895288d9d 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -623,7 +623,7 @@ static int finish_cpu(unsigned int cpu)
 	 */
 	if (mm != &init_mm)
 		idle->active_mm = &init_mm;
-	mmdrop(mm);
+	mmdrop_lazy_tlb(mm);
 	return 0;
 }
 
diff --git a/kernel/exit.c b/kernel/exit.c
index 15dc2ec80c46..1a4608d765e4 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -537,7 +537,7 @@ static void exit_mm(void)
 		return;
 	sync_mm_rss(mm);
 	mmap_read_lock(mm);
-	mmgrab(mm);
+	mmgrab_lazy_tlb(mm);
 	BUG_ON(mm != current->active_mm);
 	/* more a memory barrier than a real lock */
 	task_lock(current);
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 7424a1839e9a..e4bc32a88866 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1410,6 +1410,11 @@ void kthread_use_mm(struct mm_struct *mm)
 	WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
 	WARN_ON_ONCE(tsk->mm);
 
+	/*
+	 * It is possible for mm to be the same as tsk->active_mm, but
+	 * we must still mmgrab(mm) and mmdrop_lazy_tlb(active_mm),
+	 * because these references are not equivalent.
+	 */
 	mmgrab(mm);
 
 	task_lock(tsk);
@@ -1433,9 +1438,9 @@ void kthread_use_mm(struct mm_struct *mm)
 	 * memory barrier after storing to tsk->mm, before accessing
 	 * user-space memory. A full memory barrier for membarrier
 	 * {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by
-	 * mmdrop().
+	 * mmdrop_lazy_tlb().
 	 */
-	mmdrop(active_mm);
+	mmdrop_lazy_tlb(active_mm);
 }
 EXPORT_SYMBOL_GPL(kthread_use_mm);
 
@@ -1463,10 +1468,13 @@ void kthread_unuse_mm(struct mm_struct *mm)
 	local_irq_disable();
 	tsk->mm = NULL;
 	membarrier_update_current_mm(NULL);
+	mmgrab_lazy_tlb(mm);
 	/* active_mm is still 'mm' */
 	enter_lazy_tlb(mm, tsk);
 	local_irq_enable();
 	task_unlock(tsk);
+
+	mmdrop(mm);
 }
 EXPORT_SYMBOL_GPL(kthread_unuse_mm);
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e838feb6adc5..495f9a021de9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5189,13 +5189,14 @@ static struct rq *finish_task_switch(struct task_struct *prev)
 	 * rq->curr, before returning to userspace, so provide them here:
 	 *
 	 * - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED, implicitly
-	 *   provided by mmdrop(),
+	 *   provided by mmdrop_lazy_tlb(),
 	 * - a sync_core for SYNC_CORE.
 	 */
 	if (mm) {
 		membarrier_mm_sync_core_before_usermode(mm);
-		mmdrop_sched(mm);
+		mmdrop_lazy_tlb_sched(mm);
 	}
+
 	if (unlikely(prev_state == TASK_DEAD)) {
 		if (prev->sched_class->task_dead)
 			prev->sched_class->task_dead(prev);
@@ -5252,9 +5253,9 @@ context_switch(struct rq *rq, struct task_struct *prev,
 
 	/*
 	 * kernel -> kernel   lazy + transfer active
-	 *   user -> kernel   lazy + mmgrab() active
+	 *   user -> kernel   lazy + mmgrab_lazy_tlb() active
 	 *
-	 * kernel ->   user   switch + mmdrop() active
+	 * kernel ->   user   switch + mmdrop_lazy_tlb() active
 	 *   user ->   user   switch
 	 */
 	if (!next->mm) {                                // to kernel
@@ -5262,7 +5263,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 
 		next->active_mm = prev->active_mm;
 		if (prev->mm)                           // from user
-			mmgrab(prev->active_mm);
+			mmgrab_lazy_tlb(prev->active_mm);
 		else
 			prev->active_mm = NULL;
 	} else {                                        // to user
@@ -5279,7 +5280,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 		lru_gen_use_mm(next->mm);
 
 		if (!prev->mm) {                        // from kernel
-			/* will mmdrop() in finish_task_switch(). */
+			/* will mmdrop_lazy_tlb() in finish_task_switch(). */
 			rq->prev_mm = prev->active_mm;
 			prev->active_mm = NULL;
 		}
@@ -9916,7 +9917,7 @@ void __init sched_init(void)
 	/*
 	 * The boot idle thread does lazy MMU switching as well:
 	 */
-	mmgrab(&init_mm);
+	mmgrab_lazy_tlb(&init_mm);
 	enter_lazy_tlb(&init_mm, current);
 
 	/*
-- 
2.37.2