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SubjectRe: block: ioc->refcount accessed twice in put_io_context()?
On 2011-04-11 03:54, Shaohua Li wrote:
> 2011/4/10 Paul Bolle <pebolle@tiscali.nl>:
>> 0) Looking for clues to solve a problem I ran into, I noticed something
>> odd in block/blk-ioc.c:put_io_context(). It seems it accesses the atomic
>> variable ioc->refcount twice in a way which suggests things might race.
>>
>> 1) Code is more exact than words, so this (entirely untested) patch to
>> solve this possible race might describe better what this is all about:
>>
>> @@ -33,12 +33,16 @@ static void cfq_dtor(struct io_context *ioc)
>> */
>> int put_io_context(struct io_context *ioc)
>> {
>> + int new;
>> +
>> if (ioc == NULL)
>> return 1;
>>
>> - BUG_ON(atomic_long_read(&ioc->refcount) == 0);
>> + new = atomic_long_dec_return(&ioc->refcount);
>> +
>> + BUG_ON(new < 0);
>>
>> - if (atomic_long_dec_and_test(&ioc->refcount)) {
>> + if (new == 0) {
>> rcu_read_lock();
>> cfq_dtor(ioc);
>> rcu_read_unlock();
>>
> so you hit this line?
> BUG_ON(atomic_long_read(&ioc->refcount) == 0);
> this suggests something else is already wrong, you should fix that.

Indeed, there is nothing wrong with having the BUG_ON() there first and
doing the decrement later. If the BUG_ON() is hit, then it's not a race
conditon - it's a plain bug in the code.

--
Jens Axboe



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