Re: [PATCH v2] mmc: core: fix race condition in mmc_wait_data_done

[Ulf Hansson]

On 28 August 2015 at 16:01, Shawn Lin <shawn.lin@xxxxxxxxxxxxxx> wrote:
> From: Jialing Fu <jlfu@xxxxxxxxxxx>
>
> The following panic is captured in ker3.14, but the issue still exists
> in latest kernel.
> ---------------------------------------------------------------------
> [   20.738217] c0 3136 (Compiler) Unable to handle kernel NULL pointer dereference
> at virtual address 00000578
> ......
> [   20.738499] c0 3136 (Compiler) PC is at _raw_spin_lock_irqsave+0x24/0x60
> [   20.738527] c0 3136 (Compiler) LR is at _raw_spin_lock_irqsave+0x20/0x60
> [   20.740134] c0 3136 (Compiler) Call trace:
> [   20.740165] c0 3136 (Compiler) [<ffffffc0008ee900>] _raw_spin_lock_irqsave+0x24/0x60
> [   20.740200] c0 3136 (Compiler) [<ffffffc0000dd024>] __wake_up+0x1c/0x54
> [   20.740230] c0 3136 (Compiler) [<ffffffc000639414>] mmc_wait_data_done+0x28/0x34
> [   20.740262] c0 3136 (Compiler) [<ffffffc0006391a0>] mmc_request_done+0xa4/0x220
> [   20.740314] c0 3136 (Compiler) [<ffffffc000656894>] sdhci_tasklet_finish+0xac/0x264
> [   20.740352] c0 3136 (Compiler) [<ffffffc0000a2b58>] tasklet_action+0xa0/0x158
> [   20.740382] c0 3136 (Compiler) [<ffffffc0000a2078>] __do_softirq+0x10c/0x2e4
> [   20.740411] c0 3136 (Compiler) [<ffffffc0000a24bc>] irq_exit+0x8c/0xc0
> [   20.740439] c0 3136 (Compiler) [<ffffffc00008489c>] handle_IRQ+0x48/0xac
> [   20.740469] c0 3136 (Compiler) [<ffffffc000081428>] gic_handle_irq+0x38/0x7c
> ----------------------------------------------------------------------
> Because in SMP, "mrq" has race condition between below two paths:
> path1: CPU0: <tasklet context>
>   static void mmc_wait_data_done(struct mmc_request *mrq)
>   {
>      mrq->host->context_info.is_done_rcv = true;
>      //
>      // If CPU0 has just finished "is_done_rcv = true" in path1, and at
>      // this moment, IRQ or ICache line missing happens in CPU0.
>      // What happens in CPU1 (path2)?
>      //
>      // If the mmcqd thread in CPU1(path2) hasn't entered to sleep mode:
>      // path2 would have chance to break from wait_event_interruptible
>      // in mmc_wait_for_data_req_done and continue to run for next
>      // mmc_request (mmc_blk_rw_rq_prep).
>      //
>      // Within mmc_blk_rq_prep, mrq is cleared to 0.
>      // If below line still gets host from "mrq" as the result of
>      // compiler, the panic happens as we traced.
>      wake_up_interruptible(&mrq->host->context_info.wait);
>   }
>
> path2: CPU1: <The mmcqd thread runs mmc_queue_thread>
>   static int mmc_wait_for_data_req_done(...
>   {
>      ...
>      while (1) {
>            wait_event_interruptible(context_info->wait,
>                    (context_info->is_done_rcv ||
>                     context_info->is_new_req));
>            static void mmc_blk_rw_rq_prep(...
>            {
>            ...
>            memset(brq, 0, sizeof(struct mmc_blk_request));
>
> This issue happens very coincidentally; however adding mdelay(1) in
> mmc_wait_data_done as below could duplicate it easily.
>
>    static void mmc_wait_data_done(struct mmc_request *mrq)
>    {
>      mrq->host->context_info.is_done_rcv = true;
> +    mdelay(1);
>      wake_up_interruptible(&mrq->host->context_info.wait);
>     }
>
> At runtime, IRQ or ICache line missing may just happen at the same place
> of the mdelay(1).
>
> This patch gets the mmc_context_info at the beginning of function, it can
> avoid this race condition.
>
> Fixes: 2220eedfd7ae ("mmc: fix async request mechanism ....")
> Signed-off-by: Jialing Fu <jlfu@xxxxxxxxxxx>
>
> Signed-off-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> Tested-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>

Thanks, I actually applied v1 last Friday but updated the changelog
according to v2 just now. It's applied for next.

Kind regards
Uffe

> ---
>
> Changes in v2:
> - add fixes tag
>
>  drivers/mmc/core/core.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index 664b617..0520064 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -358,8 +358,10 @@ EXPORT_SYMBOL(mmc_start_bkops);
>   */
>  static void mmc_wait_data_done(struct mmc_request *mrq)
>  {
> -       mrq->host->context_info.is_done_rcv = true;
> -       wake_up_interruptible(&mrq->host->context_info.wait);
> +       struct mmc_context_info *context_info = &mrq->host->context_info;
> +
> +       context_info->is_done_rcv = true;
> +       wake_up_interruptible(&context_info->wait);
>  }
>
>  static void mmc_wait_done(struct mmc_request *mrq)
> --
> 2.3.7
>
>
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