On 3/15/19 5:20 PM, Christoph Hellwig wrote: > On Fri, Mar 15, 2019 at 04:57:36PM +0800, Jianchao Wang wrote: >> Hi Jens >> >> As we know, there is a risk of accesing stale requests when iterate >> in-flight requests with tags->rqs[] and this has been talked in following >> thread, >> [1] https://urldefense.proofpoint.com/v2/url?u=https-3A__marc.info_-3Fl-3Dlinux-2Dscsi-26m-3D154511693912752-26w-3D2&d=DwICAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=7WdAxUBeiTUTCy8v-7zXyr4qk7sx26ATvfo6QSTvZyQ&m=CydqJPTf4FUrfs7ipUc2chm2jGuNuDVn_onIetKEehM&s=ZQ7RfO6-737-t5kQv7SFlXMhIdpwn_AxJI93d6c-nj0&e= >> [2] https://urldefense.proofpoint.com/v2/url?u=https-3A__marc.info_-3Fl-3Dlinux-2Dblock-26m-3D154526189023236-26w-3D2&d=DwICAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=7WdAxUBeiTUTCy8v-7zXyr4qk7sx26ATvfo6QSTvZyQ&m=CydqJPTf4FUrfs7ipUc2chm2jGuNuDVn_onIetKEehM&s=EBV1M5p4mE8jZ5ZD1ecU5kMbJ9EtbpVJoc7Tqolrsc8&e= > > I'd rather take one step back and figure out why we are iterating > the busy requests. There really shouldn't be any reason why a driver > is even doings that (vs some error handling helpers in the core > block code that can properly synchronize). > A typical scene is blk_mq_in_flight, blk_mq_get_request blk_mq_in_flight -> blk_mq_get_tag -> blk_mq_queue_tag_busy_iter -> bt_for_each -> bt_iter -> rq = taags->rqs[] -> rq->q //---> get a stale request -> blk_mq_rq_ctx_init -> data->hctx->tags->rqs[rq->tag] = rq This stale request maybe something that has been freed due to io scheduler is detached or a q using a shared tagset is gone. And also the blk_mq_timeout_work could use it to pick up the expired request. The driver would also use it to requeue the in-flight requests when the device is dead. Compared with adding more synchronization, using static_rqs[] directly maybe simpler :) Thanks Jianchao
