Subject: [PATCH 2/8] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode

From: Nicholas Piggin <[email-masked]>
Date: Sun, 29 Nov 2020 02:01:35 +1000

And get rid of the generic sync_core_before_usermode facility. This is
functionally a no-op in the core scheduler code, but it also catches

This helper is the wrong way around I think. The idea that membarrier
state requires a core sync before returning to user is the easy one
that does not need hiding behind membarrier calls. The gap in core
synchronization due to x86's sysret/sysexit and lazy tlb mode, is the
tricky detail that is better put in x86 lazy tlb code.

Consider if an arch did not synchronize core in switch_mm either, then
membarrier_mm_sync_core_before_usermode would be in the wrong place
but arch specific mmu context functions would still be the right place.
There is also a exit_lazy_tlb case that is not covered by this call, which
could be a bugs (kthread use mm the membarrier process's mm then context
switch back to the process without switching mm or lazy mm switch).

This makes lazy tlb code a bit more modular.

Signed-off-by: Nicholas Piggin <[email-masked]>
---
 .../membarrier-sync-core/arch-support.txt     |  6 ++++-
 arch/x86/include/asm/mmu_context.h            | 27 +++++++++++++++++++
 include/linux/sched/mm.h                      | 14 ----------
 kernel/cpu.c                                  |  4 ++-
 kernel/sched/core.c                           | 16 +++++------
 5 files changed, 42 insertions(+), 25 deletions(-)

diff --git a/Documentation/features/sched/membarrier-sync-core/arch-support.txt b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
index 47e6903f47a5..0763a63a7097 100644
--- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt
+++ b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
@@ -5,6 +5,10 @@
 #
 # Architecture requirements
 #
+# If your architecture returns to user-space through non-core-serializing
+# instructions, you need to ensure these are done in switch_mm and exit_lazy_tlb
+# (if lazy tlb switching is implemented).
+#
 # * arm/arm64/powerpc
 #
 # Rely on implicit context synchronization as a result of exception return
@@ -24,7 +28,7 @@
 # instead on write_cr3() performed by switch_mm() to provide core serialization
 # after changing the current mm, and deal with the special case of kthread ->
 # uthread (temporarily keeping current mm into active_mm) by issuing a
-# sync_core_before_usermode() in that specific case.
+# serializing instruction in exit_lazy_mm() in that specific case.
 #
     -----------------------
     |         arch |status|
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 36afcbea6a9f..8094893254f1 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -6,12 +6,14 @@
 #include <linux/atomic.h>
 #include <linux/mm_types.h>
 #include <linux/pkeys.h>
+#include <linux/sched/mm.h>
 
 #include <trace/events/tlb.h>
 
 #include <asm/tlbflush.h>
 #include <asm/paravirt.h>
 #include <asm/debugreg.h>
+#include <asm/sync_core.h>
 
 extern atomic64_t last_mm_ctx_id;
 
@@ -94,6 +96,31 @@ static inline void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
 #define enter_lazy_tlb enter_lazy_tlb
 extern void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk);
 
+#ifdef CONFIG_MEMBARRIER
+/*
+ * Ensure that a core serializing instruction is issued before returning
+ * to user-mode, if a SYNC_CORE was requested. x86 implements return to
+ * user-space through sysexit, sysrel, and sysretq, which are not core
+ * serializing.
+ *
+ * See the membarrier comment in finish_task_switch as to why this is done
+ * in exit_lazy_tlb.
+ */
+#define exit_lazy_tlb exit_lazy_tlb
+static inline void exit_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
+{
+	/* Switching mm is serializing with write_cr3 */
+        if (tsk->mm != mm)
+                return;
+
+        if (likely(!(atomic_read(&mm->membarrier_state) &
+                     MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE)))
+                return;
+
+	sync_core_before_usermode();
+}
+#endif
+
 /*
  * Init a new mm.  Used on mm copies, like at fork()
  * and on mm's that are brand-new, like at execve().
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index d5ece7a9a403..2c6bcdf76d99 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -7,7 +7,6 @@
 #include <linux/sched.h>
 #include <linux/mm_types.h>
 #include <linux/gfp.h>
-#include <linux/sync_core.h>
 
 /*
  * Routines for handling mm_structs
@@ -335,16 +334,6 @@ enum {
 #include <asm/membarrier.h>
 #endif
 
-static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
-{
-	if (current->mm != mm)
-		return;
-	if (likely(!(atomic_read(&mm->membarrier_state) &
-		     MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE)))
-		return;
-	sync_core_before_usermode();
-}
-
 extern void membarrier_exec_mmap(struct mm_struct *mm);
 
 #else
@@ -358,9 +347,6 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 static inline void membarrier_exec_mmap(struct mm_struct *mm)
 {
 }
-static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
-{
-}
 #endif
 
 #endif /* _LINUX_SCHED_MM_H */
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6ff2578ecf17..134688d79589 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -572,7 +572,9 @@ static int finish_cpu(unsigned int cpu)
 
 	/*
 	 * idle_task_exit() will have switched to &init_mm, now
-	 * clean up any remaining active_mm state.
+	 * clean up any remaining active_mm state. exit_lazy_tlb
+	 * is not done, if an arch did any accounting in these
+	 * functions it would have to be added.
 	 */
 	if (mm != &init_mm)
 		idle->active_mm = &init_mm;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index dcc46039ade5..e4e8cebd82e2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3620,22 +3620,19 @@ static struct rq *finish_task_switch(struct task_struct *prev)
 	kcov_finish_switch(current);
 
 	fire_sched_in_preempt_notifiers(current);
+
 	/*
 	 * When switching through a kernel thread, the loop in
 	 * membarrier_{private,global}_expedited() may have observed that
 	 * kernel thread and not issued an IPI. It is therefore possible to
 	 * schedule between user->kernel->user threads without passing though
-	 * switch_mm(). Membarrier requires a barrier after storing to
-	 * rq->curr, before returning to userspace, so provide them here:
-	 *
-	 * - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED, implicitly
-	 *   provided by mmdrop(),
-	 * - a sync_core for SYNC_CORE.
+	 * switch_mm(). Membarrier requires a full barrier after storing to
+	 * rq->curr, before returning to userspace, for
+	 * {PRIVATE,GLOBAL}_EXPEDITED. This is implicitly provided by mmdrop().
 	 */
-	if (mm) {
-		membarrier_mm_sync_core_before_usermode(mm);
+	if (mm)
 		mmdrop(mm);
-	}
+
 	if (unlikely(prev_state == TASK_DEAD)) {
 		if (prev->sched_class->task_dead)
 			prev->sched_class->task_dead(prev);
@@ -6689,6 +6686,7 @@ void idle_task_exit(void)
 	BUG_ON(current != this_rq()->idle);
 
 	if (mm != &init_mm) {
+		/* enter_lazy_tlb is not done because we're about to go down */
 		switch_mm(mm, &init_mm, current);
 		finish_arch_post_lock_switch();
 	}
-- 
2.23.0