Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Aug 4, 2020, at 10:34 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Tue, Jul 28, 2020 at 12:00:09PM -0400, Mathieu Desnoyers wrote:
>> 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.
> 
> I'm still wonder what the exact failure case is though; exit_mm() is on
> the exit path (as the name very much implies) and the thread is about to
> die. The context switch that follows guarantees a full barrier before we
> run anything else again.

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)

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com