Hi, 在 2022/12/20 4:55, Tejun Heo 写道:
Hello, On Sat, Dec 17, 2022 at 11:09:04AM +0800, Yu Kuai wrote:From: Yu Kuai <yukuai3@xxxxxxxxxx> iocost is initialized when it's configured the first time, and iocost initializing can race with del_gendisk(), which will cause null pointer dereference: t1 t2 ioc_qos_write blk_iocost_init rq_qos_add del_gendisk rq_qos_exit //iocost is removed from q->roqs blkcg_activate_policy pd_init_fn ioc_pd_init ioc = q_to_ioc(blkg->q) //can't find iocost and return null And iolatency is about to switch to the same lazy initialization. This patchset fix this problem by synchronize rq_qos_add() and blkcg_activate_policy() with rq_qos_exit().So, the patchset seems a bit overly complicated to me. What do you think about the following? * These init/exit paths are super cold path, just protecting them with a global mutex is probably enough. If we encounter a scalability problem, it's easy to fix down the line. * If we're synchronizing this with a mutex anyway, no need to grab the queue_lock, right? rq_qos_add/del/exit() can all just hold the mutex. * And we can keep the state tracking within rq_qos. When rq_qos_exit() is called, mark it so that future adds will fail - be that a special ->next value a queue flag or whatever.
Yes, that sounds good. BTW, queue_lock is also used to protect pd_alloc_fn/pd_init_fn，and we found that blkcg_activate_policy() is problematic: blkcg_activate_policy spin_lock_irq(&q->queue_lock); list_for_each_entry_reverse(blkg, &q->blkg_list pd_alloc_fn(GFP_NOWAIT | __GFP_NOWARN,...) -> failed spin_unlock_irq(&q->queue_lock); // release queue_lock here is problematic, this will cause pd_offline_fn called without pd_init_fn. pd_alloc_fn(__GFP_NOWARN,...) If we are using a mutex to protect rq_qos ops, it seems the right thing to do do also using the mutex to protect blkcg_policy ops, and this problem can be fixed because mutex can be held to alloc memroy with GFP_KERNEL. What do you think? Thanks, Kuai
Thanks.
