Hi Bart, On 18/2/3 00:21, Bart Van Assche wrote: > On Fri, 2018-02-02 at 09:02 +0800, Joseph Qi wrote: >> We triggered this race when using single queue. I'm not sure if it >> exists in multi-queue. > > Regarding the races between modifying the queue_lock pointer and the code that > uses that pointer, I think the following construct in blk_cleanup_queue() is > sufficient to avoid races between the queue_lock pointer assignment and the > code > that executes concurrently with blk_cleanup_queue(): > > spin_lock_irq(lock); > if (q->queue_lock != &q->__queue_lock) > q->queue_lock = &q->__queue_lock; > spin_unlock_irq(lock); > IMO, the race also exists.
blk_cleanup_queue blkcg_print_blkgs spin_lock_irq(lock) (1) spin_lock_irq(blkg->q->queue_lock) (2,5) q->queue_lock = &q->__queue_lock (3) spin_unlock_irq(lock) (4) spin_unlock_irq(blkg->q->queue_lock) (6) (1) take driver lock; (2) busy loop for driver lock; (3) override driver lock with internal lock; (4) unlock driver lock; (5) can take driver lock now; (6) but unlock internal lock. If we get blkg->q->queue_lock to local first like blk_cleanup_queue, it indeed can fix the different lock use in lock/unlock. But since blk_cleanup_queue has overridden queue lock to internal lock now, I'm afraid we couldn't still use driver lock in blkcg_print_blkgs. Thanks, Joseph > In other words, I think that this patch series should be sufficient to address > all races between .queue_lock assignments and the code that uses that pointer. > > Thanks, > > Bart. >