Re: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations

["Huang\, Ying" <ying.huang@intel.com>]

Tim Chen <tim.c.chen@linux.intel.com> writes:

> On 2/11/19 10:47 PM, Huang, Ying wrote:
>> Andrea Parri <andrea.parri@amarulasolutions.com> writes:
>> 
>>>>> +	if (!si)
>>>>> +		goto bad_nofile;
>>>>> +
>>>>> +	preempt_disable();
>>>>> +	if (!(si->flags & SWP_VALID))
>>>>> +		goto unlock_out;
>>>>
>>>> After Hugh alluded to barriers, it seems the read of SWP_VALID could be
>>>> reordered with the write in preempt_disable at runtime.  Without smp_mb()
>>>> between the two, couldn't this happen, however unlikely a race it is?
>>>>
>>>> CPU0                                CPU1
>>>>
>>>> __swap_duplicate()
>>>>     get_swap_device()
>>>>         // sees SWP_VALID set
>>>>                                    swapoff
>>>>                                        p->flags &= ~SWP_VALID;
>>>>                                        spin_unlock(&p->lock); // pair w/ smp_mb
>>>>                                        ...
>>>>                                        stop_machine(...)
>>>>                                        p->swap_map = NULL;
>>>>         preempt_disable()
>>>>     read NULL p->swap_map
>>>
>>>
>>> I don't think that that smp_mb() is necessary.  I elaborate:
>>>
>>> An important piece of information, I think, that is missing in the
>>> diagram above is the stopper thread which executes the work queued
>>> by stop_machine().  We have two cases to consider, that is,
>>>
>>>   1) the stopper is "executed before" the preempt-disable section
>>>
>>> 	CPU0
>>>
>>> 	cpu_stopper_thread()
>>> 	...
>>> 	preempt_disable()
>>> 	...
>>> 	preempt_enable()
>>>
>>>   2) the stopper is "executed after" the preempt-disable section
>>>
>>> 	CPU0
>>>
>>> 	preempt_disable()
>>> 	...
>>> 	preempt_enable()
>>> 	...
>>> 	cpu_stopper_thread()
>>>
>>> Notice that the reads from p->flags and p->swap_map in CPU0 cannot
>>> cross cpu_stopper_thread().  The claim is that CPU0 sees SWP_VALID
>>> unset in (1) and that it sees a non-NULL p->swap_map in (2).
>>>
>>> I consider the two cases separately:
>>>
>>>   1) CPU1 unsets SPW_VALID, it locks the stopper's lock, and it
>>>      queues the stopper work; CPU0 locks the stopper's lock, it
>>>      dequeues this work, and it reads from p->flags.
>>>
>>>      Diagrammatically, we have the following MP-like pattern:
>>>
>>> 	CPU0				CPU1
>>>
>>> 	lock(stopper->lock)		p->flags &= ~SPW_VALID
>>> 	get @work			lock(stopper->lock)
>>> 	unlock(stopper->lock)		add @work
>>> 	reads p->flags 			unlock(stopper->lock)
>>>
>>>      where CPU0 must see SPW_VALID unset (if CPU0 sees the work
>>>      added by CPU1).
>>>
>>>   2) CPU0 reads from p->swap_map, it locks the completion lock,
>>>      and it signals completion; CPU1 locks the completion lock,
>>>      it checks for completion, and it writes to p->swap_map.
>>>
>>>      (If CPU0 doesn't signal the completion, or CPU1 doesn't see
>>>      the completion, then CPU1 will have to iterate the read and
>>>      to postpone the control-dependent write to p->swap_map.)
>>>
>>>      Diagrammatically, we have the following LB-like pattern:
>>>
>>> 	CPU0				CPU1
>>>
>>> 	reads p->swap_map		lock(completion)
>>> 	lock(completion)		read completion->done
>>> 	completion->done++		unlock(completion)
>>> 	unlock(completion)		p->swap_map = NULL
>>>
>>>      where CPU0 must see a non-NULL p->swap_map if CPU1 sees the
>>>      completion from CPU0.
>>>
>>> Does this make sense?
>> 
>> Thanks a lot for detailed explanation!
>
> This is certainly a non-trivial explanation of why memory barrier is not
> needed.  Can we put it in the commit log and mention something in
> comments on why we don't need memory barrier?

Good idea!  Will do this.

Best Regards,
Huang, Ying

> Thanks.
>
> Tim