Re: [PATCH] ppc32: fix destroy_context() race condition

[Guillaume Autran <gautran@mrv.com>]

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Eugene, destroy_context(..) can be called from other place outside of 
the scheduler and switch_mm(...). See mmdrop(...) for example.
In many of those cases, the task will run with preemption enable.

Guillaume.


Eugene Surovegin wrote:

>On Tue, Jul 12, 2005 at 10:23:51AM -0400, Guillaume Autran wrote:
>  
>
>>Fix for a race condition when a task gets preempted by another task
>>while executing the destroy_context(...) in a FEW_CONTEXTS environment.
>>mm->context == NO_CONTEXT but the context_map may indicate all contexts
>>are in use.
>>The solution to this problem is to disable kernel preemption while
>>destroying a MMU context.
>>
>>Signed-off-by: Guillaume Autran <gautran@mrv.com>
>>
>>---
>>
>>diff -Nru a/include/asm-ppc/mmu_context.h b/include/asm-ppc/mmu_context.h
>>--- a/include/asm-ppc/mmu_context.h	2005-06-17 15:48:29.000000000 -0400
>>+++ b/include/asm-ppc/mmu_context.h	2005-07-05 08:58:46.000000000 -0400
>>@@ -149,6 +149,7 @@
>>*/
>>static inline void destroy_context(struct mm_struct *mm)
>>{
>>+	preempt_disable();
>>	if (mm->context != NO_CONTEXT) {
>>		clear_bit(mm->context, context_map);
>>		mm->context = NO_CONTEXT;
>>@@ -156,6 +157,7 @@
>>		atomic_inc(&nr_free_contexts);
>>#endif
>>	}
>>+	preempt_enable();
>>}
>>
>>static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>>
>>    
>>
>
>Could you explain, why this patch is needed?
>
>AFAIK, destroy_context() is only called from switch_mm(), which in 
>turn is called from schedule() with preemption already disabled. If 
>not, IMHO we have bigger problems, and "fixing" destroy_context() will 
>only hide such problems.
>
>  
>

-- 
=======================================
Guillaume Autran
Senior Software Engineer
MRV Communications, Inc.
Tel: (978) 952-4932 office
E-mail: gautran@mrv.com
======================================= 


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