On Wed, Dec 02, 2020 at 11:18:31AM +0000, John Garry wrote: > On 02/12/2020 03:31, Ming Lei wrote: > > On Tue, Dec 01, 2020 at 09:02:18PM +0800, John Garry wrote: > > > It has been reported many times that a use-after-free can be intermittently > > > found when iterating busy requests: > > > > > > - https://lore.kernel.org/linux-block/8376443a-ec1b-0cef-8244-ed584b96fa96@xxxxxxxxxx/ > > > - https://lore.kernel.org/linux-block/5c3ac5af-ed81-11e4-fee3-f92175f14daf@xxxxxxx/T/#m6c1ac11540522716f645d004e2a5a13c9f218908 > > > - https://lore.kernel.org/linux-block/04e2f9e8-79fa-f1cb-ab23-4a15bf3f64cc@xxxxxxxxx/ > > > > > > The issue is that when we switch scheduler or change queue nr_requests, > > > the driver tagset may keep references to the stale requests. > > > > > > As a solution, clean up any references to those requests in the driver > > > tagset when freeing. This is done with a cmpxchg to make safe any race > > > with setting the driver tagset request from another queue. > > > > > > Signed-off-by: John Garry <john.garry@xxxxxxxxxx> > > > -- > > > Set as RFC as I need to test more. And not sure on solution method, as > > > Bart had another idea. > > > > > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c > > > index d1eafe2c045c..9b042c7036b3 100644 > > > --- a/block/blk-mq-sched.c > > > +++ b/block/blk-mq-sched.c > > > @@ -621,7 +621,7 @@ void blk_mq_sched_free_requests(struct request_queue *q) > > > queue_for_each_hw_ctx(q, hctx, i) { > > > if (hctx->sched_tags) > > > - blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i); > > > + blk_mq_free_rqs_ext(q->tag_set, hctx->sched_tags, i, hctx->tags); > > > } > > > } > > > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c > > > index 9c92053e704d..562db72e7d79 100644 > > > --- a/block/blk-mq-tag.c > > > +++ b/block/blk-mq-tag.c > > > @@ -576,7 +576,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx, > > > return -ENOMEM; > > > } > > > - blk_mq_free_rqs(set, *tagsptr, hctx->queue_num); > > > + blk_mq_free_rqs_ext(set, *tagsptr, hctx->queue_num, hctx->tags); > > > blk_mq_free_rq_map(*tagsptr, flags); > > > *tagsptr = new; > > > } else { > > > diff --git a/block/blk-mq.c b/block/blk-mq.c > > > index 55bcee5dc032..f3aad695cd25 100644 > > > --- a/block/blk-mq.c > > > +++ b/block/blk-mq.c > > > @@ -2271,8 +2271,8 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio) > > > return BLK_QC_T_NONE; > > > } > > > -void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags, > > > - unsigned int hctx_idx) > > > +void blk_mq_free_rqs_ext(struct blk_mq_tag_set *set, struct blk_mq_tags *tags, > > > + unsigned int hctx_idx, struct blk_mq_tags *references) > > > { > > > struct page *page; > > > @@ -2281,10 +2281,13 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags, > > > for (i = 0; i < tags->nr_tags; i++) { > > > struct request *rq = tags->static_rqs[i]; > > > + int j; > > > if (!rq) > > > continue; > > > set->ops->exit_request(set, rq, hctx_idx); > > > + for (j = 0; references && j < references->nr_tags; j++) > > > + cmpxchg(&references->rqs[j], rq, 0); > > > > Seems you didn't address the comment in the following link: > > > > https://lore.kernel.org/linux-block/10331543-9e45-ae63-8cdb-17e5a2a3b7ef@xxxxxxxxxx/ > > > > The request to be freed may still be refered in another path, such as blk_mq_queue_tag_busy_iter > > or blk_mq_tagset_busy_iter(), and cmpxchg() doesn't drain/wait for other refers. > > > > Hi Ming, > > Yeah, so I said that was another problem which you mentioned there, which > I'm not addressing, but I don't think that I'm making thing worse here. The thing is that this patch does not fix the issue completely. > > So AFAICS, the blk-mq/sched code doesn't wait for any "readers" to be > finished, such as those running blk_mq_queue_tag_busy_iter or > blk_mq_tagset_busy_iter() in another context. > > So how about the idea of introducing some synchronization primitive, such as > semaphore, which those "readers" must grab and release at start and end (of > iter), to ensure the requests are not freed during the iteration? It looks good, however devil is in details, please make into patch for review. thanks, Ming
