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Date:   Fri, 14 Aug 2020 12:43:56 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Nicholas Piggin <npiggin@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org
Subject: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)

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@...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: Thomas Gleixner <tglx@...utronix.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: linux-mm@...ck.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

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