[PATCH v3 3/8] mm/vma: rename is_vma_write_only(), separate out shared refcount put

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

The is_vma_writer_only() function is misnamed - this isn't determining if
there is only a write lock, as it checks for the presence of the
VM_REFCNT_EXCLUDE_READERS_FLAG.

Really, it is checking to see whether readers are excluded, with a
possibility of a false positive in the case of a detachment (there we
expect the vma->vm_refcnt to eventually be set to
VM_REFCNT_EXCLUDE_READERS_FLAG, whereas for an attached VMA we expect it to
eventually be set to VM_REFCNT_EXCLUDE_READERS_FLAG + 1).

Rename the function accordingly.

Relatedly, we use a finnicky __refcount_dec_and_test() primitive directly
in vma_refcount_put(), using the old value to determine what the reference
count ought to be after the operation is complete (ignoring racing
reference count adjustments).

Wrap this into a __vma_refcount_put() function, which we can then utilise
in vma_mark_detached() and thus keep the refcount primitive usage
abstracted.

Also adjust comments, removing duplicative comments covered elsewhere and
adding more to aid understanding.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/mmap_lock.h | 62 +++++++++++++++++++++++++++++++--------
 mm/mmap_lock.c            | 18 +++++-------
 2 files changed, 57 insertions(+), 23 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index a764439d0276..0b3614aadbb4 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -122,15 +122,27 @@ static inline void vma_lock_init(struct vm_area_struct *vma, bool reset_refcnt)
 	vma->vm_lock_seq = UINT_MAX;
 }
 
-static inline bool is_vma_writer_only(int refcnt)
+/**
+ * are_readers_excluded() - Determine whether @refcnt describes a VMA which has
+ * excluded all VMA read locks.
+ * @refcnt: The VMA reference count obtained from vm_area_struct->vm_refcnt.
+ *
+ * We may be raced by other readers temporarily incrementing the reference
+ * count, though the race window is very small, this might cause spurious
+ * wakeups.
+ *
+ * In the case of a detached VMA, we may incorrectly indicate that readers are
+ * excluded when one remains, because in that scenario we target a refcount of
+ * VM_REFCNT_EXCLUDE_READERS_FLAG, rather than the attached target of
+ * VM_REFCNT_EXCLUDE_READERS_FLAG + 1.
+ *
+ * However, the race window for that is very small so it is unlikely.
+ *
+ * Returns: true if readers are excluded, false otherwise.
+ */
+static inline bool are_readers_excluded(int refcnt)
 {
 	/*
-	 * With a writer and no readers, refcnt is VM_REFCNT_EXCLUDE_READERS_FLAG
-	 * if the vma is detached and (VM_REFCNT_EXCLUDE_READERS_FLAG + 1) if it is
-	 * attached. Waiting on a detached vma happens only in
-	 * vma_mark_detached() and is a rare case, therefore most of the time
-	 * there will be no unnecessary wakeup.
-	 *
 	 * See the comment describing the vm_area_struct->vm_refcnt field for
 	 * details of possible refcnt values.
 	 */
@@ -138,18 +150,42 @@ static inline bool is_vma_writer_only(int refcnt)
 		refcnt <= VM_REFCNT_EXCLUDE_READERS_FLAG + 1;
 }
 
+static inline bool __vma_refcount_put(struct vm_area_struct *vma, int *refcnt)
+{
+	int oldcnt;
+	bool detached;
+
+	detached = __refcount_dec_and_test(&vma->vm_refcnt, &oldcnt);
+	if (refcnt)
+		*refcnt = oldcnt - 1;
+	return detached;
+}
+
+/**
+ * vma_refcount_put() - Drop reference count in VMA vm_refcnt field due to a
+ * read-lock being dropped.
+ * @vma: The VMA whose reference count we wish to decrement.
+ *
+ * If we were the last reader, wake up threads waiting to obtain an exclusive
+ * lock.
+ */
 static inline void vma_refcount_put(struct vm_area_struct *vma)
 {
-	/* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
+	/* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt. */
 	struct mm_struct *mm = vma->vm_mm;
-	int oldcnt;
+	int refcnt;
+	bool detached;
 
 	rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
-	if (!__refcount_dec_and_test(&vma->vm_refcnt, &oldcnt)) {
 
-		if (is_vma_writer_only(oldcnt - 1))
-			rcuwait_wake_up(&mm->vma_writer_wait);
-	}
+	detached = __vma_refcount_put(vma, &refcnt);
+	/*
+	 * __vma_enter_locked() may be sleeping waiting for readers to drop
+	 * their reference count, so wake it up if we were the last reader
+	 * blocking it from being acquired.
+	 */
+	if (!detached && are_readers_excluded(refcnt))
+		rcuwait_wake_up(&mm->vma_writer_wait);
 }
 
 /*
diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index 75dc098aea14..ebacb57e5f16 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -130,25 +130,23 @@ EXPORT_SYMBOL_GPL(__vma_start_write);
 
 void vma_mark_detached(struct vm_area_struct *vma)
 {
+	bool detached;
+
 	vma_assert_write_locked(vma);
 	vma_assert_attached(vma);
 
 	/*
-	 * We are the only writer, so no need to use vma_refcount_put().
-	 * The condition below is unlikely because the vma has been already
-	 * write-locked and readers can increment vm_refcnt only temporarily
-	 * before they check vm_lock_seq, realize the vma is locked and drop
-	 * back the vm_refcnt. That is a narrow window for observing a raised
-	 * vm_refcnt.
-	 *
 	 * See the comment describing the vm_area_struct->vm_refcnt field for
 	 * details of possible refcnt values.
 	 */
-	if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt))) {
+	detached = __vma_refcount_put(vma, NULL);
+	if (unlikely(!detached)) {
 		/* Wait until vma is detached with no readers. */
 		if (__vma_enter_locked(vma, true, TASK_UNINTERRUPTIBLE)) {
-			bool detached;
-
+			/*
+			 * Once this is complete, no readers can increment the
+			 * reference count, and the VMA is marked detached.
+			 */
 			__vma_exit_locked(vma, &detached);
 			WARN_ON_ONCE(!detached);
 		}
-- 
2.52.0