On Wed, Sep 08, 2021 at 04:29:57PM +0800, yukuai (C) wrote: > On 2021/09/08 16:00, Ming Lei wrote: > > On Wed, Sep 08, 2021 at 03:37:06PM +0800, yukuai (C) wrote: > > > On 2021/09/08 15:30, Ming Lei wrote: > > > > > > > > +put_req: > > > > > + if (req) > > > > > + blk_mq_put_rq_ref(req); > > > > > return ret ? ERR_PTR(ret) : cmd; > > > > > > > > After the request's refcnt is dropped, it can be freed immediately, then > > > > one stale command is returned to caller. > > > > > > Hi, Ming > > > > > > It's right this patch leave this problem. Please take a look at patch 3 > > > and patch 4, the problem should be fixed with these patches. > > > > Not see it is addressed in patch 3 & 4, and it is one obvious fault in > > patch 2, please fix it from beginning by moving the refcnt drop > > into recv_work(). > > Hi, Ming > > With patch 3 & 4: > > if nbd_read_stat() return a valid cmd, then the refcnt should not drop > to 0 before blk_mq_complete_request() in recv_work(). The valid cmd won't be timed out just between nbd_read_stat() and calling blk_mq_complete_request()? Yeah, the issue is addressed by patch 4, then please put 2 after 3 & 4, and suggest to add comment why request ref won't drop to zero in recv_work(). > > if nbd_read_stat() failed, it won't be a problem if the request is freed > immediately when refcnt is dropped in nbd_read_stat(). > > That's why I said that the problem will be fixed. > > BTW, if we move the refcnt drop into recv_work, we can only do that if > nbd_read_stat() return a valid cmd. If we get a valid rq and failed in > the following checkings in nbd_read_stat(), it's better to drop the > refcnt in nbd_read_stat(). The usual pattern is to drop the refcnt on when the object isn't used any more, so it is perfectly fine to hold the ref until that time. Thanks, Ming
