From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, Will Deacon <will@kernel.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
Andy Lutomirski <luto@amacapital.net>,
Andrew Morton <akpm@linux-foundation.org>,
Alan Stern <stern@rowland.harvard.edu>,
Nicholas Piggin <npiggin@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Thomas Gleixner <tglx@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-mm@kvack.org
Subject: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)
Date: Fri, 14 Aug 2020 12:43:56 -0400 [thread overview]
Message-ID: <20200814164358.4783-2-mathieu.desnoyers@efficios.com> (raw)
In-Reply-To: <20200814164358.4783-1-mathieu.desnoyers@efficios.com>
exit_mm should issue memory barriers after user-space memory accesses,
before clearing current->mm, to order user-space memory accesses
performed prior to exit_mm before clearing tsk->mm, which has the
effect of skipping the membarrier private expedited IPIs.
The membarrier system call can be issued concurrently with do_exit
if we have thread groups created with CLONE_VM but not CLONE_THREAD.
Here is the scenario I have in mind:
Two thread groups are created, A and B. Thread group B is created by
issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD.
Let's assume we have a single thread within each thread group (Thread A
and Thread B).
The AFAIU we can have:
Userspace variables:
int x = 0, y = 0;
CPU 0 CPU 1
Thread A Thread B
(in thread group A) (in thread group B)
x = 1
barrier()
y = 1
exit()
exit_mm()
current->mm = NULL;
r1 = load y
membarrier()
skips CPU 0 (no IPI) because its current mm is NULL
r2 = load x
BUG_ON(r1 == 1 && r2 == 0)
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-mm@kvack.org
---
Changes since v1:
- Use smp_mb__after_spinlock rather than smp_mb.
- Document race scenario in commit message.
---
kernel/exit.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/kernel/exit.c b/kernel/exit.c
index 733e80f334e7..fe64e6e28dd5 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -475,6 +475,14 @@ static void exit_mm(void)
BUG_ON(mm != current->active_mm);
/* more a memory barrier than a real lock */
task_lock(current);
+ /*
+ * When a thread 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__after_spinlock();
current->mm = NULL;
mmap_read_unlock(mm);
enter_lazy_tlb(mm, current);
--
2.11.0
next prev parent reply other threads:[~2020-08-14 16:44 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-14 16:43 [RFC PATCH 0/3] sched: membarrier updates Mathieu Desnoyers
2020-08-14 16:43 ` Mathieu Desnoyers [this message]
2020-08-16 15:23 ` [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2) Boqun Feng
2020-09-24 15:01 ` Mathieu Desnoyers
2020-08-14 16:43 ` [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v2) Mathieu Desnoyers
2020-08-16 15:29 ` Boqun Feng
2020-08-24 15:27 ` Mathieu Desnoyers
2020-08-25 2:06 ` Boqun Feng
2020-09-24 15:26 ` Mathieu Desnoyers
2020-08-14 16:43 ` [RFC PATCH 3/3] sched: membarrier: document memory ordering scenarios Mathieu Desnoyers
[not found] ` <20200816070932.10752-1-hdanton@sina.com>
2020-08-16 21:05 ` [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v2) Mathieu Desnoyers
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