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SubjectRe: [PATCH 1/3] sched: fix exit_mm vs membarrier (v4)
On Tue, Oct 20, 2020 at 09:47:13AM -0400, Mathieu Desnoyers wrote:
> +void membarrier_update_current_mm(struct mm_struct *next_mm)
> +{
> + struct rq *rq = this_rq();
> + int membarrier_state = 0;
> +
> + if (next_mm)
> + membarrier_state = atomic_read(&next_mm->membarrier_state);
> + if (READ_ONCE(rq->membarrier_state) == membarrier_state)
> + return;
> + WRITE_ONCE(rq->membarrier_state, membarrier_state);
> +}

This is suspisioucly similar to membarrier_switch_mm().

Would something like so make sense?

---
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -206,14 +206,7 @@ void membarrier_exec_mmap(struct mm_stru

void membarrier_update_current_mm(struct mm_struct *next_mm)
{
- struct rq *rq = this_rq();
- int membarrier_state = 0;
-
- if (next_mm)
- membarrier_state = atomic_read(&next_mm->membarrier_state);
- if (READ_ONCE(rq->membarrier_state) == membarrier_state)
- return;
- WRITE_ONCE(rq->membarrier_state, membarrier_state);
+ membarrier_switch_mm(this_rq(), NULL, next_mm);
}

static int membarrier_global_expedited(void)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d2621155393c..3d589c2ffd28 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2645,12 +2645,14 @@ static inline void membarrier_switch_mm(struct rq *rq,
struct mm_struct *prev_mm,
struct mm_struct *next_mm)
{
- int membarrier_state;
+ int membarrier_state = 0;

if (prev_mm == next_mm)
return;

- membarrier_state = atomic_read(&next_mm->membarrier_state);
+ if (next_mm)
+ membarrier_state = atomic_read(&next_mm->membarrier_state);
+
if (READ_ONCE(rq->membarrier_state) == membarrier_state)
return;

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