Messages in this thread | | | Date | Thu, 9 Aug 2018 09:49:04 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout |
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On Thu, Aug 09, 2018 at 09:30:51AM +0800, Leizhen (ThunderTown) wrote: > On 2018/8/8 18:12, Will Deacon wrote: > > 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.
Thanks. Please mention that in the commit message, because I think it's useful to know.
> >> 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.
If you can provide numbers showing that it's a problem, then we could add a helper function e.g. arm_smmu_cmdq_sync_set_msidata(arm_smmu_cmdq_ent *cmd)
> In addition, I will append a optimization patch: the adjacent CMD_SYNCs, > we only need one.
Ok, but please keep them separate, since I don't want to fix up fixes and optimisations.
Thanks,
Will
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