Subject: [PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

From: [email-masked] (Leizhen (ThunderTown))
Date: Thu, 9 Aug 2018 09:30:51 +0800

On 2018/8/8 18:12, Will Deacon wrote:
> Hi Thunder,
> 
> On Mon, Aug 06, 2018 at 08:31:29PM +0800, Zhen Lei wrote:
>> The condition "(int)(VAL - sync_idx) >= 0" to break loop in function
>> __arm_smmu_sync_poll_msi requires that sync_idx must be increased
>> monotonously according to the sequence of the CMDs in the cmdq.
>>
>> But ".msidata = atomic_inc_return_relaxed(&smmu->sync_nr)" is not protected
>> by spinlock, so the following scenarios may appear:
>> cpu0			cpu1
>> msidata=0
>> 			msidata=1
>> 			insert cmd1
>> insert cmd0
>> 			smmu execute cmd1
>> smmu execute cmd0
>> 			poll timeout, because msidata=1 is overridden by
>> 			cmd0, that means VAL=0, sync_idx=1.
> 
> Oh yuck, you're right! We probably want a CC stable on this. Did you see
> this go wrong in practice?
Just misreported and make the caller wait for a long time until TIMEOUT. It's
rare to happen, because any other CMD_SYNC during the waiting period will break
it.

> 
> One comment on your patch...
> 
>> Signed-off-by: Zhen Lei <[email-masked]>
>> ---
>>  drivers/iommu/arm-smmu-v3.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>> index 1d64710..4810f61 100644
>> --- a/drivers/iommu/arm-smmu-v3.c
>> +++ b/drivers/iommu/arm-smmu-v3.c
>> @@ -566,7 +566,7 @@ struct arm_smmu_device {
>>
>>  	int				gerr_irq;
>>  	int				combined_irq;
>> -	atomic_t			sync_nr;
>> +	u32				sync_nr;
>>
>>  	unsigned long			ias; /* IPA */
>>  	unsigned long			oas; /* PA */
>> @@ -836,7 +836,6 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>>  			cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_CS, CMDQ_SYNC_0_CS_SEV);
>>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSH, ARM_SMMU_SH_ISH);
>>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
>> -		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent->sync.msidata);
>>  		cmd[1] |= ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
>>  		break;
>>  	default:
>> @@ -947,7 +946,6 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>>  	struct arm_smmu_cmdq_ent ent = {
>>  		.opcode = CMDQ_OP_CMD_SYNC,
>>  		.sync	= {
>> -			.msidata = atomic_inc_return_relaxed(&smmu->sync_nr),
>>  			.msiaddr = virt_to_phys(&smmu->sync_count),
>>  		},
>>  	};
>> @@ -955,6 +953,8 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>>  	arm_smmu_cmdq_build_cmd(cmd, &ent);
>>
>>  	spin_lock_irqsave(&smmu->cmdq.lock, flags);
>> +	ent.sync.msidata = ++smmu->sync_nr;
>> +	cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent.sync.msidata);
> 
> I really don't like splitting this out from building the rest of the
> command. Can you just move the call to arm_smmu_cmdq_build_cmd into the
> critical section, please?
OK. I have considered that before, just worry it will increase the compition of spinlock.

In addition, I will append a optimization patch: the adjacent CMD_SYNCs, we only need one.

> Thanks,
> Will
> .

-- 
Thanks!
BestRegards