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Message-Id: <20200728160010.3314-2-mathieu.desnoyers@efficios.com>
Date: Tue, 28 Jul 2020 12:00:10 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Will Deacon <will@...nel.org>,
"Paul E . McKenney" <paulmck@...nel.org>,
Nicholas Piggin <npiggin@...il.com>,
Andy Lutomirski <luto@...capital.net>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: [RFC PATCH 2/2] sched: membarrier: cover kthread_use_mm
Add comments and memory barrier to kthread_use_mm and kthread_unuse_mm
to allow the effect of membarrier(2) to apply to kthreads accessing
user-space memory as well.
Given that no prior kthread use this guarantee and that it only affects
kthreads, adding this guarantee does not affect user-space ABI.
Refine the check in membarrier_global_expedited to exclude runqueues
running the idle thread rather than all kthreads from the IPI cpumask.
This patch applies on top of this patch from Peter Zijlstra:
"mm: fix kthread_use_mm() vs TLB invalidate" currently in Andrew
Morton's tree.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Will Deacon <will@...nel.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>
---
kernel/kthread.c | 19 +++++++++++++++++++
kernel/sched/membarrier.c | 8 ++------
2 files changed, 21 insertions(+), 6 deletions(-)
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 48925b17920e..ef2435517f14 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1258,8 +1258,19 @@ void kthread_use_mm(struct mm_struct *mm)
finish_arch_post_lock_switch();
#endif
+ /*
+ * When a kthread starts operating on an address space, the loop
+ * in membarrier_{private,global}_expedited() may not observe
+ * that tsk->mm, and not issue an IPI. Membarrier requires a
+ * 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().
+ */
if (active_mm != mm)
mmdrop(active_mm);
+ else
+ smp_mb();
to_kthread(tsk)->oldfs = get_fs();
set_fs(USER_DS);
@@ -1280,6 +1291,14 @@ void kthread_unuse_mm(struct mm_struct *mm)
set_fs(to_kthread(tsk)->oldfs);
task_lock(tsk);
+ /*
+ * When a kthread stops operating on an address space, the loop
+ * in membarrier_{private,global}_expedited() may not observe
+ * that tsk->mm, and not issue an IPI. Membarrier requires a
+ * memory barrier after accessing user-space memory, before
+ * clearing tsk->mm.
+ */
+ smp_mb();
sync_mm_rss(mm);
local_irq_disable();
tsk->mm = NULL;
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 168479a7d61b..8a294483074d 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -100,13 +100,9 @@ static int membarrier_global_expedited(void)
MEMBARRIER_STATE_GLOBAL_EXPEDITED))
continue;
- /*
- * Skip the CPU if it runs a kernel thread. The scheduler
- * leaves the prior task mm in place as an optimization when
- * scheduling a kthread.
- */
+ /* Skip the CPU if it runs the idle thread. */
p = rcu_dereference(cpu_rq(cpu)->curr);
- if (p->flags & PF_KTHREAD)
+ if (is_idle_task(p))
continue;
__cpumask_set_cpu(cpu, tmpmask);
--
2.11.0
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