iommu/vt-d: Fix lockdep splat in intel_pasid_get_entry()

The pasid_lock is used to synchronize different threads from modifying a same pasid directory entry at the same time. It causes below lockdep splat.

[   83.296538] ========================================================
[   83.296538] WARNING: possible irq lock inversion dependency detected
[   83.296539] 5.12.0-rc3+ #25 Tainted: G        W
[   83.296539] --------------------------------------------------------
[   83.296540] bash/780 just changed the state of lock:
[   83.296540] ffffffff82b29c98 (device_domain_lock){..-.}-{2:2}, at:
           iommu_flush_dev_iotlb.part.0+0x32/0x110
[   83.296547] but this lock took another, SOFTIRQ-unsafe lock in the past:
[   83.296547]  (pasid_lock){+.+.}-{2:2}
[   83.296548]

and interrupts could create inverse lock ordering between them.

[   83.296549] other info that might help us debug this:
[   83.296549] Chain exists of:
                 device_domain_lock --> &iommu->lock --> pasid_lock
[   83.296551]  Possible interrupt unsafe locking scenario:

[   83.296551]        CPU0                    CPU1
[   83.296552]        ----                    ----
[   83.296552]   lock(pasid_lock);
[   83.296553]                                local_irq_disable();
[   83.296553]                                lock(device_domain_lock);
[   83.296554]                                lock(&iommu->lock);
[   83.296554]   <Interrupt>
[   83.296554]     lock(device_domain_lock);
[   83.296555]
                *** DEADLOCK ***

Fix it by replacing the pasid_lock with an atomic exchange operation.

---
 drivers/iommu/intel/pasid.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

Log:
v1->v2:
- v1: https://lore.kernel.org/linux-iommu/[email protected]/
- Use retry to make code neat;
- Add a comment about no clear case, hence no race.

index 7a73385edcc0..f2c747e62c6a 100644
@@ -24,7 +24,6 @@
 /*
  * Intel IOMMU system wide PASID name space:
  */
-static DEFINE_SPINLOCK(pasid_lock);
 u32 intel_pasid_max_id = PASID_MAX;
 
 int vcmd_alloc_pasid(struct intel_iommu *iommu, u32 *pasid)

@@ -259,19 +258,25 @@ struct pasid_entry *intel_pasid_get_entry(struct device *dev, u32 pasid)
 	dir_index = pasid >> PASID_PDE_SHIFT;
 	index = pasid & PASID_PTE_MASK;
 
-	spin_lock(&pasid_lock);
+retry:
 	entries = get_pasid_table_from_pde(&dir[dir_index]);
 	if (!entries) {
 		entries = alloc_pgtable_page(info->iommu->node);
-		if (!entries) {
-			spin_unlock(&pasid_lock);
+		if (!entries)
 			return NULL;
-		}
 
-		WRITE_ONCE(dir[dir_index].val,
-			   (u64)virt_to_phys(entries) | PASID_PTE_PRESENT);
+		/*
+		 * The pasid directory table entry won't be freed after
+		 * allocation. No worry about the race with free and
+		 * clear. However, this entry might be populated by others
+		 * while we are preparing it. Use theirs with a retry.
+		 */
+		if (cmpxchg64(&dir[dir_index].val, 0ULL,
+			      (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {
+			free_pgtable_page(entries);
+			goto retry;
+		}
 	}
-	spin_unlock(&pasid_lock);
 
 	return &entries[index];
 }

2.25.1

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Applied, thanks.
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