[PATCH RESEND v3 07/10] mm/vma: introduce helper struct + thread through exclusive lock fns

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

It is confusing to have __vma_enter_exclusive_locked() return 0, 1 or an
error (but only when waiting for readers in TASK_KILLABLE state), and
having the return value be stored in a stack variable called 'locked' is
further confusion.

More generally, we are doing a lock of rather finnicky things during the
acquisition of a state in which readers are excluded and moving out of this
state, including tracking whether we are detached or not or whether an
error occurred.

We are implementing logic in __vma_enter_exclusive_locked() that
effectively acts as if 'if one caller calls us do X, if another then do Y',
which is very confusing from a control flow perspective.

Introducing the shared helper object state helps us avoid this, as we can
now handle the 'an error arose but we're detached' condition correctly in
both callers - a warning if not detaching, and treating the situation as if
no error arose in the case of a VMA detaching.

This also acts to help document what's going on and allows us to add some
more logical debug asserts.

Also update vma_mark_detached() to add a guard clause for the likely
'already detached' state (given we hold the mmap write lock), and add a
comment about ephemeral VMA read lock reference count increments to clarify
why we are entering/exiting an exclusive locked state here.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/mmap_lock.c | 144 +++++++++++++++++++++++++++++++------------------
 1 file changed, 91 insertions(+), 53 deletions(-)

diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index f73221174a8b..75166a43ffa4 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -46,20 +46,40 @@ EXPORT_SYMBOL(__mmap_lock_do_trace_released);
 #ifdef CONFIG_MMU
 #ifdef CONFIG_PER_VMA_LOCK

+/* State shared across __vma_[enter, exit]_exclusive_locked(). */
+struct vma_exclude_readers_state {
+	/* Input parameters. */
+	struct vm_area_struct *vma;
+	int state; /* TASK_KILLABLE or TASK_UNINTERRUPTIBLE. */
+	bool detaching;
+
+	bool detached;
+	bool exclusive; /* Are we exclusively locked? */
+};
+
 /*
  * Now that all readers have been evicted, mark the VMA as being out of the
  * 'exclude readers' state.
  *
  * Returns true if the VMA is now detached, otherwise false.
  */
-static bool __must_check __vma_exit_exclusive_locked(struct vm_area_struct *vma)
+static void __vma_exit_exclusive_locked(struct vma_exclude_readers_state *ves)
 {
-	bool detached;
+	struct vm_area_struct *vma = ves->vma;
+
+	VM_WARN_ON_ONCE(ves->detached);
+	VM_WARN_ON_ONCE(!ves->exclusive);

-	detached = refcount_sub_and_test(VM_REFCNT_EXCLUDE_READERS_FLAG,
-					 &vma->vm_refcnt);
+	ves->detached = refcount_sub_and_test(VM_REFCNT_EXCLUDE_READERS_FLAG,
+					      &vma->vm_refcnt);
 	__vma_lockdep_release_exclusive(vma);
-	return detached;
+}
+
+static unsigned int get_target_refcnt(struct vma_exclude_readers_state *ves)
+{
+	const unsigned int tgt = ves->detaching ? 0 : 1;
+
+	return tgt | VM_REFCNT_EXCLUDE_READERS_FLAG;
 }

 /*
@@ -69,30 +89,31 @@ static bool __must_check __vma_exit_exclusive_locked(struct vm_area_struct *vma)
  * Note that this function pairs with vma_refcount_put() which will wake up this
  * thread when it detects that the last reader has released its lock.
  *
- * The state parameter ought to be set to TASK_UNINTERRUPTIBLE in cases where we
- * wish the thread to sleep uninterruptibly or TASK_KILLABLE if a fatal signal
- * is permitted to kill it.
+ * The ves->state parameter ought to be set to TASK_UNINTERRUPTIBLE in cases
+ * where we wish the thread to sleep uninterruptibly or TASK_KILLABLE if a fatal
+ * signal is permitted to kill it.
  *
- * The function will return 0 immediately if the VMA is detached, and 1 once the
- * VMA has evicted all readers, leaving the VMA exclusively locked.
+ * The function sets the ves->locked parameter to true if an exclusive lock was
+ * acquired, or false if the VMA was detached or an error arose on wait.
  *
- * If the function returns 1, the caller is required to invoke
- * __vma_exit_exclusive_locked() once the exclusive state is no longer required.
+ * If the function indicates an exclusive lock was acquired via ves->exclusive
+ * (or equivalently, returning 0 with !ves->detached), the caller is required to
+ * invoke __vma_exit_exclusive_locked() once the exclusive state is no longer
+ * required.
  *
- * If state is set to something other than TASK_UNINTERRUPTIBLE, the function
- * may also return -EINTR to indicate a fatal signal was received while waiting.
+ * If ves->state is set to something other than TASK_UNINTERRUPTIBLE, the
+ * function may also return -EINTR to indicate a fatal signal was received while
+ * waiting.
  */
-static int __vma_enter_exclusive_locked(struct vm_area_struct *vma,
-		bool detaching, int state)
+static int __vma_enter_exclusive_locked(struct vma_exclude_readers_state *ves)
 {
-	int err;
-	unsigned int tgt_refcnt = VM_REFCNT_EXCLUDE_READERS_FLAG;
+	struct vm_area_struct *vma = ves->vma;
+	unsigned int tgt_refcnt = get_target_refcnt(ves);
+	int err = 0;

 	mmap_assert_write_locked(vma->vm_mm);
-
-	/* Additional refcnt if the vma is attached. */
-	if (!detaching)
-		tgt_refcnt++;
+	VM_WARN_ON_ONCE(ves->detached);
+	VM_WARN_ON_ONCE(ves->exclusive);

 	/*
 	 * If vma is detached then only vma_mark_attached() can raise the
@@ -101,37 +122,39 @@ static int __vma_enter_exclusive_locked(struct vm_area_struct *vma,
 	 * See the comment describing the vm_area_struct->vm_refcnt field for
 	 * details of possible refcnt values.
 	 */
-	if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt))
+	if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt)) {
+		ves->detached = true;
 		return 0;
+	}

 	__vma_lockdep_acquire_exclusive(vma);
 	err = rcuwait_wait_event(&vma->vm_mm->vma_writer_wait,
 		   refcount_read(&vma->vm_refcnt) == tgt_refcnt,
-		   state);
+		   ves->state);
 	if (err) {
-		if (__vma_exit_exclusive_locked(vma)) {
-			/*
-			 * The wait failed, but the last reader went away
-			 * as well. Tell the caller the VMA is detached.
-			 */
-			WARN_ON_ONCE(!detaching);
-			err = 0;
-		}
+		__vma_exit_exclusive_locked(ves);
 		return err;
 	}
-	__vma_lockdep_stat_mark_acquired(vma);

-	return 1;
+	__vma_lockdep_stat_mark_acquired(vma);
+	ves->exclusive = true;
+	return 0;
 }

 int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
 		int state)
 {
-	int locked;
+	int err;
+	struct vma_exclude_readers_state ves = {
+		.vma = vma,
+		.state = state,
+	};

-	locked = __vma_enter_exclusive_locked(vma, false, state);
-	if (locked < 0)
-		return locked;
+	err = __vma_enter_exclusive_locked(&ves);
+	if (err) {
+		WARN_ON_ONCE(ves.detached);
+		return err;
+	}

 	/*
 	 * We should use WRITE_ONCE() here because we can have concurrent reads
@@ -141,9 +164,11 @@ int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
 	 */
 	WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq);

-	/* vma should remain attached. */
-	if (locked)
-		WARN_ON_ONCE(__vma_exit_exclusive_locked(vma));
+	if (!ves.detached) {
+		__vma_exit_exclusive_locked(&ves);
+		/* VMA should remain attached. */
+		WARN_ON_ONCE(ves.detached);
+	}

 	return 0;
 }
@@ -151,7 +176,12 @@ EXPORT_SYMBOL_GPL(__vma_start_write);

 void vma_mark_detached(struct vm_area_struct *vma)
 {
-	bool detached;
+	struct vma_exclude_readers_state ves = {
+		.vma = vma,
+		.state = TASK_UNINTERRUPTIBLE,
+		.detaching = true,
+	};
+	int err;

 	vma_assert_write_locked(vma);
 	vma_assert_attached(vma);
@@ -160,18 +190,26 @@ void vma_mark_detached(struct vm_area_struct *vma)
 	 * See the comment describing the vm_area_struct->vm_refcnt field for
 	 * details of possible refcnt values.
 	 */
-	detached = __vma_refcount_put(vma, NULL);
-	if (unlikely(!detached)) {
-		/* Wait until vma is detached with no readers. */
-		if (__vma_enter_exclusive_locked(vma, true, TASK_UNINTERRUPTIBLE)) {
-			/*
-			 * Once this is complete, no readers can increment the
-			 * reference count, and the VMA is marked detached.
-			 */
-			detached = __vma_exit_exclusive_locked(vma);
-			WARN_ON_ONCE(!detached);
-		}
+	if (likely(__vma_refcount_put(vma, NULL)))
+		return;
+
+	/*
+	 * Wait until the VMA is detached with no readers. Since we hold the VMA
+	 * write lock, the only read locks that might be present are those from
+	 * threads trying to acquire the read lock and incrementing the
+	 * reference count before realising the write lock is held and
+	 * decrementing it.
+	 */
+	err = __vma_enter_exclusive_locked(&ves);
+	if (!err && !ves.detached) {
+		/*
+		 * Once this is complete, no readers can increment the
+		 * reference count, and the VMA is marked detached.
+		 */
+		__vma_exit_exclusive_locked(&ves);
 	}
+	/* If an error arose but we were detached anyway, we don't care. */
+	WARN_ON_ONCE(!ves.detached);
 }

 /*
--
2.52.0