On Thu, 29 Sep 2022 10:07:50 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > On Thu, 29 Sep 2022 14:41:26 +0200 > Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote: > > > On Mon, 5 Sept 2022 at 08:22, dinggao.pan <dinggao.pan@xxxxxxxxxx> wrote: > > > > > > Hi, > > > After applying rt patches to our 4.14 kernel and enabling preempt-rt, we met a hung task during boot caused by race condition on context_info stored in struct mmc_host. > > > From our investigation, context_info should not be changed by threads that have not claimed the host, hence the following fix. > > > > > > Any comments are much appreciated. > > > Dinggao Pan > > > > Hi Dinggao, > > > > Apologize for the delay. > > > > The 4.14 kernel is too old for me to be able to comment. In > > particular, the mmc block layer moved to blk-mq in v4.16, which means > > the path you are investigating doesn't exist any more, sorry. > > Luis (Cc'd) is still supporting the 4.14-rt kernel. It appears that I have an old email address for Luis, and it bounced. Updated with his redhat one. -- Steve > > > > > Kind regards > > Uffe > > > > > > > > From: "Dinggao Pan" <mailto:dinggao.pan@xxxxxxxxxx> > > > > > > A race condition happens under following circumstances: > > > (mmc_thread1) | (mmc_thread2) > > > mmc_issue_rq(req1) | > > > > qcnt++ for req1 | > > > host handling req1 | > > > mmc_queue_thread(req=null) | > > > > enter queue thread | > > > again, fetches blk req | > > > (return null), sets | > > > is_waiting_last_req 1 | mmc_request_fn(req1) -> set is_new_req 1 > > > | and wake_up wait_queue > > > mmc_issue_rq(req2) | > mmc_thread2 tries to claim host > > > > **qcnt++ for req2** | > > > mmc_finalize_req(req2) | > > > > should wait for req1 | > > > done but req2 return | > > > MMC_BLK_NEW_REQ | > > > due to is_new_req | > > > already set to 1 | > > > | > > > | > > > req1 done | > > > > qcnt-- for req1 | > > > mmc_issue_rq(req3) | > > > > qcnt++ for req3 | > > > req2 is not handled but qcnt is already added(noted by **), > > > thus mmc_thread1 will never release host, causing mmc_threads > > > except thread1 to hung. Fix race by moving wake_up to the front of > > > context_info update. > > > > > > Reviewed By: Yunqian Wang <mailto:yunqian.wang@xxxxxxxxxx> > > > Signed-off-by: Dinggao Pan <mailto:dinggao.pan@xxxxxxxxxx> > > > Signed-off-by: Ming Yu <mailto:ming.yu@xxxxxxxxxx> > > > --- > > > drivers/mmc/core/queue.c | 7 +++++-- > > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > > > diff --git a/drivers/mmc/core/queue.c b/drivers/mmc/core/queue.c > > > index 0a4e77a5b..58318c102 100644 > > > --- a/drivers/mmc/core/queue.c > > > +++ b/drivers/mmc/core/queue.c > > > @@ -107,6 +107,11 @@ static void mmc_request_fn(struct request_queue *q) > > > return; > > > } > > > > > > + if (mq->asleep) { > > > + wake_up_process(mq->thread); > > > + return; > > > + } > > > + > > > cntx = &mq->card->host->context_info; > > > > > > if (cntx->is_waiting_last_req) { > > > @@ -114,8 +119,6 @@ static void mmc_request_fn(struct request_queue *q) > > > wake_up_interruptible(&cntx->wait); > > > } > > > > > > - if (mq->asleep) > > > - wake_up_process(mq->thread); > > > } > > > > > > static struct scatterlist *mmc_alloc_sg(int sg_len, gfp_t gfp) > > > -- > > > 2.36.1 >
