Commit 0f02477d authored Aug 19, 2018 by Zhen Lei Committed by Will Deacon Oct 01, 2018

iommu/arm-smmu-v3: Fix unexpected CMD_SYNC timeout



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>

parent 85c7a0f1

drivers/iommu/arm-smmu-v3.c

+3 −5

Original line number	Diff line number	Diff line
		@@ -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 */
		@@ -948,14 +948,13 @@ 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);

		@@ -2180,7 +2179,6 @@ 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;