Re: [PATCH] bsg: call idr_pre_get() outside the lock.

[Naohiro Aota]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Wed, 01 Sep 2010 23:26:01 +0900
> Naohiro Aota <naota@xxxxxxxxx> wrote:
>
>> The idr_pre_get() kernel-doc says "This function should be called
>> prior to locking and calling the idr_get_new* functions.", but the
>> idr_pre_get() calling in bsg_register_queue() is put inside
>> mutex_lock(). Let's fix it.
>> 
>
> The idr_pre_get() kerneldoc is wrong.  Or at least, misleading.
>
> The way this all works is that we precharge the tree via idr_pre_get()
> and we do this outside locks so we can use GFP_KERNEL.  Then we take
> the lock (a spinlock!) and then try to add an element to the tree,
> which will consume objects from the pool which was prefilled by
> idr_pre_get().
>
> There's an obvious race here where someone else can get in and steal
> objects from the prefilled pool.  We can fix that with a retry loop:
>
>
> again:
> 	if (idr_pre_get(..., GFP_KERNEL) == NULL)
> 		return -ENOMEM;		/* We're really out-of-memory */
> 	spin_lock(lock);
> 	if (idr_get_new(...) == -EAGAIN) {
> 		spin_unlock(lock);
> 		goto again;		/* Someone stole our preallocation! */
> 	}
> 	...
>
> this way we avoid the false -ENOMEM which the race would have caused. 
> We only declare -ENOMEM when we're REALLY out of memory.
>
>
> But none of this is needed when a sleeping lock is used (as long as the
> sleeping lock isn't taken on the the VM pageout path, of course).  In
> that case we can use the sleeping lock to prevent the above race.
>
>> diff --git a/block/bsg.c b/block/bsg.c
>> index 82d5882..5fd8dd1 100644
>> --- a/block/bsg.c
>> +++ b/block/bsg.c
>> @@ -1010,13 +1010,11 @@ int bsg_register_queue(struct request_queue *q, struct device *parent,
>>  	bcd = &q->bsg_dev;
>>  	memset(bcd, 0, sizeof(*bcd));
>>  
>> -	mutex_lock(&bsg_mutex);
>> -
>>  	ret = idr_pre_get(&bsg_minor_idr, GFP_KERNEL);
>> -	if (!ret) {
>> -		ret = -ENOMEM;
>> -		goto unlock;
>> -	}
>> +	if (!ret)
>> +		return -ENOMEM;
>> +
>> +	mutex_lock(&bsg_mutex);
>>  
>>  	ret = idr_get_new(&bsg_minor_idr, bcd, &minor);
>>  	if (ret < 0)
>
> So the old code was OK.
>
> The new code, however, is not OK because it is vulnerable to the above
> race wherein another CPU or thread comes in and steals all of this
> thread's preallocation.

I see. Thank you for your comment.

> The idr_pre_get() kerneldoc is wrong.  Or at least, misleading.

Then we can fix the kerneldoc like this?

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