linux-kernel - Re: [PATCH 1/3] sched: fix exit

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Date:   Tue, 20 Oct 2020 16:36:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>, linux-mm@...ck.org
Subject: Re: [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;