Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Yang Shi <yang.shi@linux.alibaba.com>]

On 3/26/18 6:00 PM, Tetsuo Handa wrote:
> Cyrill Gorcunov wrote:
>> On Tue, Mar 27, 2018 at 06:10:09AM +0900, Tetsuo Handa wrote:
>>> On 2018/03/27 4:21, Cyrill Gorcunov wrote:
>>>> That said I think using read-lock here would be a bug.
>>> If I understand correctly, the caller can't set both fields atomically, for
>>> prctl() does not receive both fields at one call.
>>>
>>>    prctl(PR_SET_MM, PR_SET_MM_ARG_START xor PR_SET_MM_ARG_END xor PR_SET_MM_ENV_START xor PR_SET_MM_ENV_END, new value, 0, 0);
>>>
>> True, but the key moment is that two/three/four system calls can
>> run simultaneously. And while previously they are ordered by "write",
>> with read lock they are completely unordered and this is really
>> worries me.
> Yes, we need exclusive lock when updating these fields.
>
>>              To be fair I would prefer to drop this old per-field
>> interface completely. This per-field interface was rather an ugly
>> solution from my side.
> But this is userspace visible API and thus we cannot change.
>
>>> Then, I wonder whether reading arg_start|end and env_start|end atomically makes
>>> sense. Just retry reading if arg_start > env_end or env_start > env_end is fine?
>> Tetsuo, let me re-read this code tomorrow, maybe I miss something obvious.
>>
> You are not missing my point. What I thought is
>
> +retry:
> -	down_read(&mm->mmap_sem);
>   	arg_start = mm->arg_start;
>   	arg_end = mm->arg_end;
>   	env_start = mm->env_start;
>   	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
>   
> -	BUG_ON(arg_start > arg_end);
> -	BUG_ON(env_start > env_end);
> +	if (unlikely(arg_start > arg_end || env_start > env_end)) {
> +		cond_resched();
> +		goto retry;

Can't it trap into dead loop if the condition is always false?

> +	}
>
> for reading these fields.
>
> By the way, /proc/pid/ readers are serving as a canary who tells something
> mm_mmap related problem is happening. On the other hand, it is sad that
> such canary cannot be terminated by signal due to use of unkillable waits.
> I wish we can use killable waits.

I already proposed patches (https://lkml.org/lkml/2018/2/26/1197) to do 
this a few weeks ago. In the review, akpm suggested mitigate the 
mmap_sem contention instead of using killable version workaround. Then 
the preliminary unmaping by section patches 
(https://lkml.org/lkml/2018/3/20/786) were proposed. In the discussion, 
we decided to eliminate the mmap_sem abuse, this is where the patch came 
from.

Yang