[PATCH 09/23] membarrier: Fix incorrect barrier positions during exec and kthread_use_mm()

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membarrier() requires a barrier before changes to rq->curr->mm, not just
before writes to rq->membarrier_state.  Move the barrier in exec_mmap() to
the right place.  Add the barrier in kthread_use_mm() -- it was entirely
missing before.

This patch makes exec_mmap() and kthread_use_mm() use the same membarrier
hooks, which results in some code deletion.

As an added bonus, this will eliminate a redundant barrier in execve() on
arches for which spinlock acquisition is a barrier.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 fs/exec.c                 |  6 +++++-
 include/linux/sched/mm.h  |  2 --
 kernel/kthread.c          |  5 +++++
 kernel/sched/membarrier.c | 15 ---------------
 4 files changed, 10 insertions(+), 18 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 38b05e01c5bd..325dab98bc51 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1001,12 +1001,16 @@ static int exec_mmap(struct mm_struct *mm)
 	}
 
 	task_lock(tsk);
-	membarrier_exec_mmap(mm);
+	/*
+	 * membarrier() requires a full barrier before switching mm.
+	 */
+	smp_mb__after_spinlock();
 
 	local_irq_disable();
 	active_mm = tsk->active_mm;
 	tsk->active_mm = mm;
 	WRITE_ONCE(tsk->mm, mm);  /* membarrier reads this without locks */
+	membarrier_update_current_mm(mm);
 	/*
 	 * This prevents preemption while active_mm is being loaded and
 	 * it and mm are being updated, which could cause problems for
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index e107f292fc42..f1d2beac464c 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -344,8 +344,6 @@ enum {
 #include <asm/membarrier.h>
 #endif
 
-extern void membarrier_exec_mmap(struct mm_struct *mm);
-
 extern void membarrier_update_current_mm(struct mm_struct *next_mm);
 
 /*
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 3b18329f885c..18b0a2e0e3b2 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1351,6 +1351,11 @@ void kthread_use_mm(struct mm_struct *mm)
 	WARN_ON_ONCE(tsk->mm);
 
 	task_lock(tsk);
+	/*
+	 * membarrier() requires a full barrier before switching mm.
+	 */
+	smp_mb__after_spinlock();
+
 	/* Hold off tlb flush IPIs while switching mm's */
 	local_irq_disable();
 	active_mm = tsk->active_mm;
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index c38014c2ed66..44fafa6e1efd 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -277,21 +277,6 @@ static void ipi_sync_rq_state(void *info)
 	smp_mb();
 }
 
-void membarrier_exec_mmap(struct mm_struct *mm)
-{
-	/*
-	 * Issue a memory barrier before clearing membarrier_state to
-	 * guarantee that no memory access prior to exec is reordered after
-	 * clearing this state.
-	 */
-	smp_mb();
-	/*
-	 * Keep the runqueue membarrier_state in sync with this mm
-	 * membarrier_state.
-	 */
-	this_cpu_write(runqueues.membarrier_state, 0);
-}
-
 void membarrier_update_current_mm(struct mm_struct *next_mm)
 {
 	struct rq *rq = this_rq();
-- 
2.33.1





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