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SubjectRe: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)
----- On Aug 16, 2020, at 11:23 AM, Boqun Feng boqun.feng@gmail.com wrote:

> Hi Mathieu,
>
> On Fri, Aug 14, 2020 at 12:43:56PM -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.
>>
>> 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
>
> Is it accurate to say that the correctness of
> membarrier_global_expedited() relies on the observation of ->mm? Because
> IIUC membarrier_global_expedited() loop doesn't check ->mm.

Good point, I was wrong. Will instead reword as:

/*
* When a thread stops operating on an address space, the loop
* in membarrier_private_expedited() may not observe that
* tsk->mm, and the loop in membarrier_global_expedited() may
* not observe a MEMBARRIER_STATE_GLOBAL_EXPEDITED
* rq->membarrier_state, so those would not issue an IPI.
* Membarrier requires a memory barrier after accessing
* user-space memory, before clearing tsk->mm or the
* rq->membarrier_state.
*/
And I'll make sure exit_mm clears this_rq()->membarrier_state as well.

Thanks,

Mathieu

>
> Regards,
> Boqun
>
>> + * 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

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

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