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SubjectRe: [PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout
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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 <thunder.leizhen@huawei.com>
>> ---
>> 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

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