Re: [PATCH 6/7] mmc: core: protect references to host->areq with host->lock

[Grant Grundler]

Ulf,
While this patch might be correct, it's not solving the problem I
claimed and my explanation was wrong. See comments in this code
review:
    https://chromium-review.googlesource.com/#/c/170880/1//COMMIT_MSG

While I no longer see the same crash with this change in our "ToT
tree", I'm able to reproduce the original mmcqd crash on a different
kernel variant (also based on chromeos-3.4 kernel).

I think I need to review references to mqrq_prev and mqrq_cur since
those appear to be protected by mq->thread_sem and I suspect
references are happening from dw_mmc tasklet without holding this
semaphore.

apologies,
grant


On Thu, Sep 26, 2013 at 12:22 PM, Grant Grundler <grundler@xxxxxxxxxxxx> wrote:
> Races between host->areq being NULL or not are resulting in mmcqd
> hung_task panics. Like this one:
>
> <3>[  240.501202] INFO: task mmcqd/1:85 blocked for more than 120 seconds.
> <3>[  240.501213] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> <6>[  240.501223] mmcqd/1         D 80528020     0    85      2 0x00000000
> <5>[  240.501254] [<80528020>] (__schedule+0x614/0x780) from [<80528550>] (schedule+0x94/0x98)
> <5>[  240.501269] [<80528550>] (schedule+0x94/0x98) from [<80526270>] (schedule_timeout+0x38/0x2d0)
> <5>[  240.501284] [<80526270>] (schedule_timeout+0x38/0x2d0) from [<805283a4>] (wait_for_common+0x164/0x1a0)
> <5>[  240.501298] [<805283a4>] (wait_for_common+0x164/0x1a0) from [<805284b8>] (wait_for_completion+0x20/0x24)
> <5>[  240.501313] [<805284b8>] (wait_for_completion+0x20/0x24) from [<803d7068>] (mmc_wait_for_req_done+0x2c/0x84)
> <5>[  240.501327] [<803d7068>] (mmc_wait_for_req_done+0x2c/0x84) from [<803d81c0>] (mmc_start_req+0x60/0x120)
> <5>[  240.501342] [<803d81c0>] (mmc_start_req+0x60/0x120) from [<803e402c>] (mmc_blk_issue_rw_rq+0xa0/0x3a8)
> <5>[  240.501355] [<803e402c>] (mmc_blk_issue_rw_rq+0xa0/0x3a8) from [<803e4758>] (mmc_blk_issue_rq+0x424/0x478)
> <5>[  240.501368] [<803e4758>] (mmc_blk_issue_rq+0x424/0x478) from [<803e587c>] (mmc_queue_thread+0xb0/0x118)
> <5>[  240.501382] [<803e587c>] (mmc_queue_thread+0xb0/0x118) from [<8004d61c>] (kthread+0xa8/0xbc)
> <5>[  240.501396] [<8004d61c>] (kthread+0xa8/0xbc) from [<8000f1c8>] (kernel_thread_exit+0x0/0x8)
> <0>[  240.501407] Kernel panic - not syncing: hung_task: blocked tasks
> <5>[  240.501421] [<800150a4>] (unwind_backtrace+0x0/0x114) from [<80521920>] (dump_stack+0x20/0x24)
> <5>[  240.501434] [<80521920>] (dump_stack+0x20/0x24) from [<80521a90>] (panic+0xa8/0x1f4)
> <5>[  240.501447] [<80521a90>] (panic+0xa8/0x1f4) from [<80086d3c>] (watchdog+0x1f4/0x25c)
> <5>[  240.501459] [<80086d3c>] (watchdog+0x1f4/0x25c) from [<8004d61c>] (kthread+0xa8/0xbc)
> <5>[  240.501471] [<8004d61c>] (kthread+0xa8/0xbc) from [<8000f1c8>] (kernel_thread_exit+0x0/0x8)
>
> I was able to reproduce the mmcqd "hung task" timeout consistently
> with this fio command line on an Exynos5250 system with Toshiba HS200
> eMMC running in HS200 mode:
>         fio --name=short_randwrite --size=2G --time_based --runtime=3m \
>                 --readwrite=randwrite --numjobs=2 --bs=4k --norandommap \
>                 --ioengine=psync --direct=0 --filename=/dev/mmcblk0p5
>
> I believe the key parameters are "--numjobs=2" (or more) and "randwrite"
> workload.  Then the completions are happening around the same time as
> mmc_start_req() is referencing and/or updating host->areq.
>
> I was NOT able to consistently reproduce the problem on a similar
> Exynos 5250 system which had "engineering samples" of Samsung HS200
> capable eMMC installed. Just my clue that the timing is different
> (and the fio performance numbers are different too).
>
> Signed-off-by: Grant Grundler <grundler@xxxxxxxxxxxx>
> ---
>  drivers/mmc/core/core.c | 34 +++++++++++++++++++++++-----------
>  1 file changed, 23 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index 36cfe91..e5a9599 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -529,29 +529,40 @@ struct mmc_async_req *mmc_start_req(struct mmc_host *host,
>  {
>         int saved_err = 0;
>         int start_err = 0;
> -       struct mmc_async_req *saved_areq = host->areq;
> +       struct mmc_async_req *saved_areq;
> +       unsigned long flags;
>
> -       if (!saved_areq && !areq)
> -               /* Nothing to do...some code is polling. */
> +       spin_lock_irqsave(&host->lock, flags);
> +       saved_areq = host->areq;
> +       if (!saved_areq && !areq) {
> +               /* Nothing? Code is racing to harvest a completion. */
> +               spin_unlock_irqrestore(&host->lock, flags);
>                 goto set_error;
> +       }
>
>         /* Prepare a new request */
>         if (areq)
>                 mmc_pre_req(host, areq->mrq, !saved_areq);
>
>         if (saved_areq) {
> +               /* This thread owns this IO (saved_areq) for now. */
> +               host->areq = NULL;
> +               spin_unlock_irqrestore(&host->lock, flags);
> +
>                 saved_err = mmc_wait_for_data_req_done(host, saved_areq->mrq,   areq);
>                 if (saved_err == MMC_BLK_NEW_REQUEST) {
> -                       /*
> -                        * The previous request was not completed,
> -                        * nothing to return
> -                        */
> +                       spin_lock_irqsave(&host->lock, flags);
> +                       BUG_ON(host->areq != NULL);
> +
> +                       /* Not completed. Don't report it. */
> +                       host->areq = saved_areq;
>                         saved_areq = NULL;
> +                       saved_err = 0;
> +                       spin_unlock_irqrestore(&host->lock, flags);
>                         goto set_error;
>                 }
> -               /*
> -                * Check BKOPS urgency for each R1 response
> -                */
> +
> +               /* Check BKOPS urgency for each R1 response */
>                 if (host->card && mmc_card_mmc(host->card) &&
>                     ((mmc_resp_type(saved_areq->mrq->cmd) == MMC_RSP_R1) ||
>                      (mmc_resp_type(saved_areq->mrq->cmd) == MMC_RSP_R1B)) &&
> @@ -562,11 +573,12 @@ struct mmc_async_req *mmc_start_req(struct mmc_host *host,
>         /* Don't start something new if previous one failed. */
>         if (!saved_err && areq) {
>                 start_err = __mmc_start_data_req(host, areq->mrq);
> +
>                 /* Cancel a prepared request if it was not started. */
>                 if (start_err) {
>                         mmc_post_req(host, areq->mrq, -EINVAL);
>                         host->areq = NULL;
> -               } else
> +               } else
>                         host->areq = areq;
>         }
>
> --
> 1.8.4
>
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