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Re: [PATCH V2 02/02] MIPS: Init new mmu_context for each possible CPU to

To: Huacai Chen <chenhc@lemote.com>
Subject: Re: [PATCH V2 02/02] MIPS: Init new mmu_context for each possible CPU to avoid memory corruption
From: David Daney <ddaney.cavm@gmail.com>
Date: Wed, 20 Mar 2013 16:16:07 -0700
Cc: Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, Fuxin Zhang <zhangfx@lemote.com>, Zhangjin Wu <wuzhangjin@gmail.com>
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On 03/17/2013 05:50 AM, Huacai Chen wrote:
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.

Signed-off-by: Huacai Chen <chenhc@lemote.com>


We were seeing the same crashes, this patch set seems to fix the problem.

Acked-by: David Daney <david.daney@cavium.com>

---
  arch/mips/include/asm/mmu_context.h |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/mips/include/asm/mmu_context.h 
b/arch/mips/include/asm/mmu_context.h
index e81d719..49d220c 100644
--- a/arch/mips/include/asm/mmu_context.h
+++ b/arch/mips/include/asm/mmu_context.h
@@ -133,7 +133,7 @@ init_new_context(struct task_struct *tsk, struct mm_struct 
*mm)
  {
        int i;

-       for_each_online_cpu(i)
+       for_each_possible_cpu(i)
                cpu_context(i, mm) = 0;

        return 0;



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