Subject: [PATCH] ARM: pl330: Fix a race condition

From: [email-masked] (Javi Merino)
Date: Tue, 20 Sep 2011 14:36:24 +0100

On 19/09/11 19:07, Jassi Brar wrote:
> On Mon, Sep 19, 2011 at 10:41 PM, Javi Merino <[email-masked]> wrote:
>> If two requests have been submitted and one of them is running, if you
>> call pl330_chan_ctrl(ch_id, PL330_OP_START), there's a window of time
>> between the spin_lock_irqsave() and the _state() check in which the
>> running transaction may finish.  In that case, we don't receive the
>> interrupt (because they are disabled), but _start() sees that the DMA
>> is stopped, so it starts it.  The problem is that it sends the
>> transaction that has just finished again, because pl330_update()
>> hasn't mark it as done yet.
>>
>> This patch moves the _state() check out of the critical section, which
>> removes the race condition.  It also treats PL330_STATE_COMPLETING as
>> still executing, because that introduces another race condition now
>> that we call _state() with interrupts enabled.  Namely, if we read the
>> state as "completing" and the DMA sends the interrupt before we
>> disable interrupts, pl330_update() starts the next transaction and
>> returns.  Then the _start() in pl330_chan_ctrl() will patiently wait
>> until the just issued transaction finishes (because the state we read
>> was PL330_STATE_COMPLETING) and when it does, it _trigger()s the same
>> transaction again.
>>
>> Signed-off-by: Javi Merino <[email-masked]>
>> Cc: Jassi Brar <[email-masked]>
>> ---
>>  arch/arm/common/pl330.c |   12 +++++++-----
>>  1 files changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm/common/pl330.c b/arch/arm/common/pl330.c
>> index 97912fa..26b5615 100644
>> --- a/arch/arm/common/pl330.c
>> +++ b/arch/arm/common/pl330.c
>> @@ -936,9 +936,9 @@ static bool _trigger(struct pl330_thread *thrd)
>>        return true;
>>  }
>>
>> -static bool _start(struct pl330_thread *thrd)
>> +static bool _start(struct pl330_thread *thrd, u32 state)
>>  {
>> -       switch (_state(thrd)) {
>> +       switch (state) {
>>        case PL330_STATE_FAULT_COMPLETING:
>>                UNTIL(thrd, PL330_STATE_FAULTING | PL330_STATE_KILLING);
>>
>> @@ -949,7 +949,6 @@ static bool _start(struct pl330_thread *thrd)
>>                _stop(thrd);
>>
>>        case PL330_STATE_KILLING:
>> -       case PL330_STATE_COMPLETING:
>>                UNTIL(thrd, PL330_STATE_STOPPED)
>>
>>        case PL330_STATE_STOPPED:
>> @@ -961,6 +960,7 @@ static bool _start(struct pl330_thread *thrd)
>>        case PL330_STATE_UPDTPC:
>>        case PL330_STATE_CACHEMISS:
>>        case PL330_STATE_EXECUTING:
>> +       case PL330_STATE_COMPLETING:
>>                return true;
>>
>>        case PL330_STATE_WFE: /* For RESUME, nothing yet */
>> @@ -1471,7 +1471,7 @@ int pl330_update(const struct pl330_info *pi)
>>                        MARK_FREE(rqdone);
>>
>>                        /* Get going again ASAP */
>> -                       _start(thrd);
>> +                       _start(thrd, _state(thrd));
>>
>>                        /* For now, just make a list of callbacks to be done */
>>                        list_add_tail(&rqdone->rqd, &pl330->req_done);
>> @@ -1510,12 +1510,14 @@ int pl330_chan_ctrl(void *ch_id, enum pl330_chan_op op)
>>        struct pl330_dmac *pl330;
>>        unsigned long flags;
>>        int ret = 0, active;
>> +       u32 dma_state;
>>
>>        if (!thrd || thrd->free || thrd->dmac->state == DYING)
>>                return -EINVAL;
>>
>>        pl330 = thrd->dmac;
>>
>> +       dma_state = _state(thrd);
>>        spin_lock_irqsave(&pl330->lock, flags);
>>
>>        switch (op) {
>> @@ -1546,7 +1548,7 @@ int pl330_chan_ctrl(void *ch_id, enum pl330_chan_op op)
>>
>>                /* Start the next */
>>        case PL330_OP_START:
>> -               if (!_start(thrd))
>> +               if (!_start(thrd, dma_state))
>>                        ret = -EIO;
>>                break;
>>
>> --
> 
> IIUIC your race scenario should be taken care of simply by doing...
> 
> diff --git a/arch/arm/common/pl330.c b/arch/arm/common/pl330.c
> index 97912fa..7129cfb 100644
> --- a/arch/arm/common/pl330.c
> +++ b/arch/arm/common/pl330.c
> @@ -1546,7 +1546,7 @@ int pl330_chan_ctrl(void *ch_id, enum pl330_chan_op op)
> 
>  		/* Start the next */
>  	case PL330_OP_START:
> -		if (!_start(thrd))
> +		if (!_thrd_active(thrd) && !_start(thrd))
>  			ret = -EIO;
>  		break;
> 

My first reaction was this was just moving the race condition, but
thinking about it carefully, it looks like a better (simpler) solution.

> Could you please test if it fixes the issue?

Yes, I'll kick off runs later today and see how it goes.  I don't have a
way to reproduce the bug consistently, I just do lots of DMA
transactions and eventually, I end up hitting it.  I'll run it overnight
and see if it fixes it or not.

Cheers,
Javi