[PATCH AUTOSEL 4.19 022/209] iommu/arm-smmu-v3: Fix unexpected CMD_SYNC timeout

Sasha Levin
2019-11-13T02:27:40Z

From: Zhen Lei <thunder.leizhen@huawei.com>

[ Upstream commit 0f02477d16980938a84aba8688a4e3a303306116 ]

The condition break condition of:

(int)(VAL - sync_idx) >= 0

in the __arm_smmu_sync_poll_msi() polling loop requires that sync_idx
must be increased monotonically according to the sequence of the CMDs in
the cmdq.

However, since the msidata is populated using atomic_inc_return_relaxed()
before taking the command-queue spinlock, then the following scenario
can occur:

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.

This is not a functional problem, since the caller will eventually either
timeout or exit due to another CMD_SYNC, however it's clearly not what
the code is supposed to be doing. Fix it, by incrementing the sequence
count with the command-queue lock held, allowing us to drop the atomic
operations altogether.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
[will: dropped the specialised cmd building routine for now]
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/iommu/arm-smmu-v3.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index 40fbf20d69e5a..2ab7100bcff12 100644
--- a/drivers/iommu/arm-smmu-v3.c
+++ b/drivers/iommu/arm-smmu-v3.c
@@ -567,7 +567,7 @@ 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 */
@@ -964,14 +964,13 @@ 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),
},
};

- arm_smmu_cmdq_build_cmd(cmd, &ent);
-
spin_lock_irqsave(&smmu->cmdq.lock, flags);
+ ent.sync.msidata = ++smmu->sync_nr;
+ arm_smmu_cmdq_build_cmd(cmd, &ent);
arm_smmu_cmdq_insert_cmd(smmu, cmd);
spin_unlock_irqrestore(&smmu->cmdq.lock, flags);

@@ -2196,7 +2195,6 @@ static int arm_smmu_init_structures(struct arm_smmu_device *smmu)
{
int ret;

- atomic_set(&smmu->sync_nr, 0);
ret = arm_smmu_init_queues(smmu);
if (ret)
return ret;
--
2.20.1