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

[Shawn Lin]

On 2015/8/28 11:13, Shawn Lin 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);
>      }

Hi, ulf

We find this bug on Intel-C3230RK platform for very small probability.

Whereas I can easily reproduce this case if I add a mdelay(1) or  longer 
delay as Jialing did.

This patch seems useful to me. Should we push it forward? :)


> 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.
>
> Signed-off-by: Jialing Fu <jlfu@xxxxxxxxxxx>
> Tested-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> ---
>
>   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)


--
Best Regards
Shawn Lin

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