Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)

[Mathieu Desnoyers]

----- On Aug 16, 2020, at 11:23 AM, Boqun Feng boqun.feng@xxxxxxxxx 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@xxxxxxxxxxxx>
>> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>> Cc: Will Deacon <will@xxxxxxxxxx>
>> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
>> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
>> Cc: linux-mm@xxxxxxxxx
>> ---
>> 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