[PATCH] MIPS: We need to clear MMU contexts of all other processes when asid_cache(cpu) wraps to 0.

[Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>]

On 07/11/2016 11:07 AM, James Hogan wrote:
> Hi Leonid,
>
> On Mon, Jul 11, 2016 at 11:02:00AM -0700, Leonid Yegoshin wrote:
>> On 07/10/2016 06:04 AM, yhb@ruijie.com.cn wrote:
>>> Subject: [PATCH] MIPS: We need to clear MMU contexts of all other processes
>>>    when asid_cache(cpu) wraps to 0.
>>>
>>> Suppose that asid_cache(cpu) wraps to 0 every n days.
>>> case 1:
>>> (1)Process 1 got ASID 0x101.
>>> (2)Process 1 slept for n days.
>>> (3)asid_cache(cpu) wrapped to 0x101, and process 2 got ASID 0x101.
>>> (4)Process 1 is woken,and ASID of process 1 is same as ASID of process 2.
>>>
>>> case 2:
>>> (1)Process 1 got ASID 0x101 on CPU 1.
>>> (2)Process 1 migrated to CPU 2.
>>> (3)Process 1 migrated to CPU 1 after n days.
>>> (4)asid_cache on CPU 1 wrapped to 0x101, and process 2 got ASID 0x101.
>>> (5)Process 1 is scheduled, and ASID of process 1 is same as ASID of process 2.
>>>
>>> So we need to clear MMU contexts of all other processes when asid_cache(cpu) wraps to 0.
>>>
>>> Signed-off-by: yhb <yhb@ruijie.com.cn>
>>>
>> I think a more clear description should be given here - there is no
>> indication that wrap happens over 32bit integer.
>>
>> And taking into account "n days" frequency - can we just kill all local
>> ASIDs in all processes (additionally to local_flush_tlb_all) and enforce
>> reassignment if wrap happens? It should be a very rare event, you are
>> first to hit this.
>>
>> It seems to be some localized stuff in get_new_mmu_context() instead of
>> widespread patching.
> That is what this patch does, but to do so it appears you need to lock
> the other tasks one by one, and that must be doable from a context
> switch, i.e. hardirq context, and that requires the task lock to be of
> the _irqsave variant, hence the widespread changes and the relatively
> tiny MIPS change hidden in the middle.
>
Not exactly. The change must be done only for local CPU which executes 
at the moment get_new_mmu_context(). Just prevent preemption here and 
change of cpu_context(THIS_CPU,...) can be done safely - other CPUs 
don't do anything with this variable besides killing it (writing 0 to it).

You can look into flush_tlb_mm() for example how it is cleared for 
single memory map.

We have a macro to safely walk all processes, right? (don't remember 
it's name).