NFS: Fix races nfs_page_group_destroy() vs nfs_destroy_unlinked_subrequests()

When a subrequest is being detached from the subgroup, we want to
ensure that it is not holding the group lock, or in the process
of waiting for the group lock.

Fixes: 5b2b5187fa ("NFS: Fix nfs_page_group_destroy() and nfs_lock_and_join_requests() race cases")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/pagelist.c

@@ -132,9 +132,44 @@ nfs_async_iocounter_wait(struct rpc_task *task, struct nfs_lock_context *l_ctx)
 }
 EXPORT_SYMBOL_GPL(nfs_async_iocounter_wait);
 
+/*
+ * nfs_page_set_headlock - set the request PG_HEADLOCK
+ * @req: request that is to be locked
+ *
+ * this lock must be held when modifying req->wb_head
+ *
+ * return 0 on success, < 0 on error
+ */
+int
+nfs_page_set_headlock(struct nfs_page *req)
+{
+	if (!test_and_set_bit(PG_HEADLOCK, &req->wb_flags))
+		return 0;
+
+	set_bit(PG_CONTENDED1, &req->wb_flags);
+	smp_mb__after_atomic();
+	return wait_on_bit_lock(&req->wb_flags, PG_HEADLOCK,
+				TASK_UNINTERRUPTIBLE);
+}
+
+/*
+ * nfs_page_clear_headlock - clear the request PG_HEADLOCK
+ * @req: request that is to be locked
+ */
+void
+nfs_page_clear_headlock(struct nfs_page *req)
+{
+	smp_mb__before_atomic();
+	clear_bit(PG_HEADLOCK, &req->wb_flags);
+	smp_mb__after_atomic();
+	if (!test_bit(PG_CONTENDED1, &req->wb_flags))
+		return;
+	wake_up_bit(&req->wb_flags, PG_HEADLOCK);
+}
+
 /*
  * nfs_page_group_lock - lock the head of the page group
- * @req - request in group that is to be locked
+ * @req: request in group that is to be locked
  *
  * this lock must be held when traversing or modifying the page
  * group list
@@ -144,36 +179,24 @@ EXPORT_SYMBOL_GPL(nfs_async_iocounter_wait);
 int
 nfs_page_group_lock(struct nfs_page *req)
 {
-	struct nfs_page *head = req->wb_head;
+	int ret;
 
-	WARN_ON_ONCE(head != head->wb_head);
-
-	if (!test_and_set_bit(PG_HEADLOCK, &head->wb_flags))
-		return 0;
-
-	set_bit(PG_CONTENDED1, &head->wb_flags);
-	smp_mb__after_atomic();
-	return wait_on_bit_lock(&head->wb_flags, PG_HEADLOCK,
-				TASK_UNINTERRUPTIBLE);
+	ret = nfs_page_set_headlock(req);
+	if (ret || req->wb_head == req)
+		return ret;
+	return nfs_page_set_headlock(req->wb_head);
 }
 
 /*
  * nfs_page_group_unlock - unlock the head of the page group
- * @req - request in group that is to be unlocked
+ * @req: request in group that is to be unlocked
  */
 void
 nfs_page_group_unlock(struct nfs_page *req)
 {
-	struct nfs_page *head = req->wb_head;
-
-	WARN_ON_ONCE(head != head->wb_head);
-
-	smp_mb__before_atomic();
-	clear_bit(PG_HEADLOCK, &head->wb_flags);
-	smp_mb__after_atomic();
-	if (!test_bit(PG_CONTENDED1, &head->wb_flags))
-		return;
-	wake_up_bit(&head->wb_flags, PG_HEADLOCK);
+	if (req != req->wb_head)
+		nfs_page_clear_headlock(req->wb_head);
+	nfs_page_clear_headlock(req);
 }
 
 /*

fs/nfs/write.c

@@ -428,22 +428,28 @@ nfs_destroy_unlinked_subrequests(struct nfs_page *destroy_list,
 		destroy_list = (subreq->wb_this_page == old_head) ?
 				   NULL : subreq->wb_this_page;
 
+		/* Note: lock subreq in order to change subreq->wb_head */
+		nfs_page_set_headlock(subreq);
 		WARN_ON_ONCE(old_head != subreq->wb_head);
 
 		/* make sure old group is not used */
 		subreq->wb_this_page = subreq;
+		subreq->wb_head = subreq;
 
 		clear_bit(PG_REMOVE, &subreq->wb_flags);
 
 		/* Note: races with nfs_page_group_destroy() */
 		if (!kref_read(&subreq->wb_kref)) {
 			/* Check if we raced with nfs_page_group_destroy() */
-			if (test_and_clear_bit(PG_TEARDOWN, &subreq->wb_flags))
+			if (test_and_clear_bit(PG_TEARDOWN, &subreq->wb_flags)) {
+				nfs_page_clear_headlock(subreq);
 				nfs_free_request(subreq);
+			} else
+				nfs_page_clear_headlock(subreq);
 			continue;
 		}
+		nfs_page_clear_headlock(subreq);
 
-		subreq->wb_head = subreq;
 		nfs_release_request(old_head);
 
 		if (test_and_clear_bit(PG_INODE_REF, &subreq->wb_flags)) {

include/linux/nfs_page.h

@@ -142,6 +142,8 @@ extern	void nfs_unlock_and_release_request(struct nfs_page *);
 extern int nfs_page_group_lock(struct nfs_page *);
 extern void nfs_page_group_unlock(struct nfs_page *);
 extern bool nfs_page_group_sync_on_bit(struct nfs_page *, unsigned int);
+extern	int nfs_page_set_headlock(struct nfs_page *req);
+extern void nfs_page_clear_headlock(struct nfs_page *req);
 extern bool nfs_async_iocounter_wait(struct rpc_task *, struct nfs_lock_context *);
 
 /*