linux-kernel - Re: [PATCH V2 02/02] MIPS: Init new mmu_context for each possible CPU to avoid memory corruption

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Date:	Wed, 20 Mar 2013 13:49:20 +0800
From:	Chen Jie <chenj@...ote.com>
To:	Huacai Chen <chenhc@...ote.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org, Fuxin Zhang <zhangfx@...ote.com>,
	Zhangjin Wu <wuzhangjin@...il.com>
Subject: Re: [PATCH V2 02/02] MIPS: Init new mmu_context for each possible CPU
 to avoid memory corruption

2013/3/17 Huacai Chen <chenhc@...ote.com>:
> Currently, init_new_context() only for each online CPU, this may cause
> memory corruption when CPU hotplug and fork() happens at the same time.
> To avoid this, we make init_new_context() cover each possible CPU.
>
> Scenario:
> 1, CPU#1 is being offline;
> 2, On CPU#0, do_fork() call dup_mm() and copy a mm_struct to the child;
> 3, On CPU#0, dup_mm() call init_new_context(), since CPU#1 is offline
>    and init_new_context() only covers the online CPUs, child has the
>    same asid as its parent on CPU#1 (however, child's asid should be 0);
> 4, CPU#1 is being online;
> 5, Now, if both parent and child run on CPU#1, memory corruption (e.g.
>    segfault, bus error, etc.) will occur.
Adds some further explanations about point 5:
5.1) The parent process runs on CPU#1, the version field of its
asid[CPU#1] and CPU#1's asid cache may match, thus the parent
process's asid[CPU#1] is used in TLB refilling routine.

5.2) Then the child process runs on CPU#1 which probably has the same
asid[CPU#1] as its parent(as pointed by point 3).

5.3) The child process may access address space of its parent.

It can also be solved by increasing version field of asid cache when
plugin a CPU,  may be a better way?



Regards,
-- Chen Jie
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