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. 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
