[PATCH 2/2] userfaultfd: fs/userfaultfd.c add more comments

[Andrea Arcangeli <aarcange@redhat.com>]

Add more commentary.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 fs/userfaultfd.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index f601f27..a519f74 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -86,7 +86,20 @@ static int userfaultfd_wake_function(wait_queue_t *wq, unsigned mode,
 		goto out;
 	ret = wake_up_state(wq->private, mode);
 	if (ret)
-		/* wake only once, autoremove behavior */
+		/*
+		 * Wake only once, autoremove behavior.
+		 *
+		 * After the effect of list_del_init is visible to the
+		 * other CPUs, the waitqueue may disappear from under
+		 * us, see the !list_empty_careful() in
+		 * handle_userfault(). try_to_wake_up() has an
+		 * implicit smp_mb__before_spinlock, and the
+		 * wq->private is read before calling the extern
+		 * function "wake_up_state" (which in turns calls
+		 * try_to_wake_up). While the spin_lock;spin_unlock;
+		 * wouldn't be enough, the smp_mb__before_spinlock is
+		 * enough to avoid an explicit smp_mb() here.
+		 */
 		list_del_init(&wq->task_list);
 out:
 	return ret;
@@ -511,6 +524,19 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
 			 * Refile this userfault from
 			 * fault_pending_wqh to fault_wqh, it's not
 			 * pending anymore after we read it.
+			 *
+			 * Use list_del() by hand (as
+			 * userfaultfd_wake_function also uses
+			 * list_del_init() by hand) to be sure nobody
+			 * changes __remove_wait_queue() to use
+			 * list_del_init() in turn breaking the
+			 * !list_empty_careful() check in
+			 * handle_userfault(). The uwq->wq.task_list
+			 * must never be empty at any time during the
+			 * refile, or the waitqueue could disappear
+			 * from under us. The "wait_queue_head_t"
+			 * parameter of __remove_wait_queue() is unused
+			 * anyway.
 			 */
 			list_del(&uwq->wq.task_list);
 			__add_wait_queue(&ctx->fault_wqh, &uwq->wq);

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