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

From: Shawn Lin
Date: Fri Aug 28 2015 - 05:53:20 EST

å 2015/8/28 16:55, Ulf Hansson åé:
On 28 August 2015 at 05:25, Shawn Lin <shawn.lin@xxxxxxxxxxxxxx> wrote:
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
at virtual address 00000578
[ 20.738499] c0 3136 (Compiler) PC is at
[ 20.738527] c0 3136 (Compiler) LR is at
[ 20.740134] c0 3136 (Compiler) Call trace:
[ 20.740165] c0 3136 (Compiler) [<ffffffc0008ee900>]
[ 20.740200] c0 3136 (Compiler) [<ffffffc0000dd024>] __wake_up+0x1c/0x54
[ 20.740230] c0 3136 (Compiler) [<ffffffc000639414>]
[ 20.740262] c0 3136 (Compiler) [<ffffffc0006391a0>]
[ 20.740314] c0 3136 (Compiler) [<ffffffc000656894>]
[ 20.740352] c0 3136 (Compiler) [<ffffffc0000a2b58>]
[ 20.740382] c0 3136 (Compiler) [<ffffffc0000a2078>]
[ 20.740411] c0 3136 (Compiler) [<ffffffc0000a24bc>] irq_exit+0x8c/0xc0
[ 20.740439] c0 3136 (Compiler) [<ffffffc00008489c>]
[ 20.740469] c0 3136 (Compiler) [<ffffffc000081428>]
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.

path2: CPU1: <The mmcqd thread runs mmc_queue_thread>
static int mmc_wait_for_data_req_done(...
while (1) {
(context_info->is_done_rcv ||
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);

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? :)

It seems like a very good idea!

Should we add a fixes tag to it?

That's cool, but how to add a fixes tag?

[Fixes] mmc: core: fix race condition in mmc_wait_data_done ? :)


Kind regards

Best Regards
Shawn Lin

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