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SubjectRe: [PATCH RFC stable 4.14 1/1] mmc: core: fix hung task caused by race condition on context_info
On Mon, 5 Sept 2022 at 08:22, dinggao.pan <dinggao.pan@horizon.ai> 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@horizon.ai>
>
>   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@horizon.ai>
> Signed-off-by: Dinggao Pan <mailto:dinggao.pan@horizon.ai>
> Signed-off-by: Ming Yu <mailto:ming.yu@horizon.ai>
> ---
> 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

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