linux-kernel - Re: [PATCH] LIB: remove unmatched write_lock() in gen_pool

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Message-ID: <4A36C55F.6060002@opengridcomputing.com>
Date:	Mon, 15 Jun 2009 17:04:15 -0500
From:	Steve Wise <swise@...ngridcomputing.com>
To:	Jiri Kosina <trivial@...nel.org>
CC:	Zygo Blaxell <zygo.blaxell@...dros.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] LIB: remove unmatched write_lock() in gen_pool_destroy


Jiri Kosina wrote:
> On Fri, 12 Jun 2009, Zygo Blaxell wrote:
>
>   
>> Fix mismatch between calls to write_lock() and write_unlock() in
>> gen_pool_destroy by removing the write_lock().
>>
>> Signed-off-by: Zygo Blaxell <zygo.blaxell@...dros.com>
>> ---
>> There is a call to write_lock() in gen_pool_destroy which is not balanced
>> by any corresponding write_unlock().  This causes problems with preemption
>> because the preemption-disable counter is incremented in the write_lock()
>> call, but never decremented by any call to write_unlock().  This bug is
>> difficult to observe in the field because only two in-tree drivers call
>> gen_pool_destroy, and one of them is non-x86 arch-specific code.
>>
>> To fix this, I have chosen removing the write_lock() over adding a
>> write_unlock() because the lock in question is inside a structure which
>> is being freed.  Any other thread that waited to acquire such a lock
>> while gen_pool_destroy was running would find itself holding a lock
>> in recently-freed or about-to-be-freed memory.  This would result in
>> memory corruption or a crash whether &pool->lock is held or not.
>>
>> Using a pool while it is in the process of being destroyed is a bug that
>> must be resolved outside of the gen_pool_destroy function.
>>
>>  lib/genalloc.c |    1 -
>>  1 files changed, 0 insertions(+), 1 deletions(-)
>>
>> diff --git a/lib/genalloc.c b/lib/genalloc.c
>> index f6d276d..eed2bdb 100644
>> --- a/lib/genalloc.c
>> +++ b/lib/genalloc.c
>> @@ -85,7 +85,6 @@ void gen_pool_destroy(struct gen_pool *pool)
>>  	int bit, end_bit;
>>  
>>  
>> -	write_lock(&pool->lock);
>>  	list_for_each_safe(_chunk, _next_chunk, &pool->chunks) {
>>  		chunk = list_entry(_chunk, struct gen_pool_chunk, next_chunk);
>>  		list_del(&chunk->next_chunk);
>> -- 
>> 1.5.6.5
>>
>>     
>
> Hi Zygo,
>
> this doesn't really qualify for trivial tree, as it introduces a 
> significant code change. Adding some CCs.
>
>   

Looks ok to me.  Its dumb to aquire the lock you're gonna free anyway.  
Maybe some BUG_ON() that sez nobody better be holding this lock?



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