On Thu, Sep 29, 2022 at 02:41:26PM +0200, Ulf Hansson 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. And the new code has the queue operations protected by a spinlock (queue_lock), which I believe is necessary to fix the issue reported here. Luis > 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 > ---end quoted text---
