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SubjectRe: [PATCH] MIPS: reset all task's asid to 0 after asid_cache(cpu) overflows
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On 3/6/2017 10:21 AM, jsun4 wrote:

>>> If asid_cache(cpu) overflows, there may be two tasks with the same
>>> asid. It is a risk that the two different tasks may have the same
>>> address space.
>>>
>>> A process will update its asid to newer version only when switch_mm()
>>> is called and matches the following condition:
>>> if ((cpu_context(cpu, next) ^ asid_cache(cpu))
>>> & asid_version_mask(cpu))
>>> get_new_mmu_context(next, cpu);
>>> If asid_cache(cpu) overflows, cpu_context(cpu,next) and asid_cache(cpu)
>>> will be reset to asid_first_version(cpu), and start a new cycle. It
>>> can result in two tasks that have the same ASID in the process list.
>>>
>>> For example, in CONFIG_CPU_MIPS32_R2, task named A's asid on CPU1 is
>>> 0x100, and has been sleeping and been not scheduled. After a long period
>>> of time, another running task named B's asid on CPU1 is 0xffffffff, and
>>> asid cached in the CPU1 is 0xffffffff too, next task named C is forked,
>>> when schedule from B to C on CPU1, asid_cache(cpu) will overflow, so C's
>>> asid on CPU1 will be 0x100 according to get_new_mmu_context(). A's asid
>>> is the same as C, if now A is rescheduled on CPU1, A's asid is not able
>>> to renew according to 'if' clause, and the local TLB entry can't be
>>> flushed too, A's address space will be the same as C.
>>>
>>> If asid_cache(cpu) overflows, all of user space task's asid on this CPU
>>> are able to set a invalid value (such as 0), it will avoid the risk.
>>>
>>> Signed-off-by: Jiwei Sun <jiwei.sun@windriver.com>
>>> ---
>>> arch/mips/include/asm/mmu_context.h | 9 ++++++++-
>>> 1 file changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/mips/include/asm/mmu_context.h b/arch/mips/include/asm/mmu_context.h
>>> index ddd57ad..1f60efc 100644
>>> --- a/arch/mips/include/asm/mmu_context.h
>>> +++ b/arch/mips/include/asm/mmu_context.h
>>> @@ -108,8 +108,15 @@ static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
>>> #else
>>> local_flush_tlb_all(); /* start new asid cycle */
>>> #endif
>>> - if (!asid) /* fix version if needed */
>>> + if (!asid) { /* fix version if needed */
>>> + struct task_struct *p;
>>> +
>>> + for_each_process(p) {
>>> + if ((p->mm))
>>
>> Why double parens?
>
> At the beginning, the code was written as following
> if ((p->mm) && (p->mm != mm))
> cpu_context(cpu, p->mm) = 0;
>
> Because cpu_context(cpu,mm) will be changed to asid_first_version(cpu) after 'for' loop,
> and in order to improve the efficiency of the loop, I deleted "&& (p->mm != mm)",
> but I forgot to delete the redundant parentheses.

Note that parens around 'p->mm' were never needed. And neither around the
right operand of &&.

> Thanks,
> Best regards,
> Jiwei

MBR, Sergei

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