Re: [PATCH 1/8] xarray: Fix premature termination of xas_for_each_marked()

[Matthew Wilcox <willy@infradead.org>]

Hi Jan,

I fixed this in a different way ... also, I added a test to the test-suite
so this shouldn't regress.  Thanks for writing the commit message ;-)

From 7e934cf5ace1dceeb804f7493fa28bb697ed3c52 Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Thu, 12 Mar 2020 17:29:11 -0400
Subject: [PATCH] xarray: Fix early termination of xas_for_each_marked

xas_for_each_marked() is using entry == NULL as a termination condition
of the iteration. When xas_for_each_marked() is used protected only by
RCU, this can however race with xas_store(xas, NULL) in the following
way:

TASK1                                   TASK2
page_cache_delete()         	        find_get_pages_range_tag()
                                          xas_for_each_marked()
                                            xas_find_marked()
                                              off = xas_find_chunk()

  xas_store(&xas, NULL)
    xas_init_marks(&xas);
    ...
    rcu_assign_pointer(*slot, NULL);
                                              entry = xa_entry(off);

And thus xas_for_each_marked() terminates prematurely possibly leading
to missed entries in the iteration (translating to missing writeback of
some pages or a similar problem).

If we find a NULL entry that has been marked, skip it (unless we're trying
to allocate an entry).

Reported-by: Jan Kara <jack@suse.cz>
CC: stable@vger.kernel.org
Fixes: ef8e5717db01 ("page cache: Convert delete_batch to XArray")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/xarray.h                       |  6 +-
 lib/xarray.c                                 |  2 +
 tools/testing/radix-tree/Makefile            |  4 +-
 tools/testing/radix-tree/iteration_check_2.c | 87 ++++++++++++++++++++
 tools/testing/radix-tree/main.c              |  1 +
 tools/testing/radix-tree/test.h              |  1 +
 6 files changed, 98 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/radix-tree/iteration_check_2.c

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index a491653d8882..14c893433139 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -1648,6 +1648,7 @@ static inline void *xas_next_marked(struct xa_state *xas, unsigned long max,
 								xa_mark_t mark)
 {
 	struct xa_node *node = xas->xa_node;
+	void *entry;
 	unsigned int offset;
 
 	if (unlikely(xas_not_node(node) || node->shift))
@@ -1659,7 +1660,10 @@ static inline void *xas_next_marked(struct xa_state *xas, unsigned long max,
 		return NULL;
 	if (offset == XA_CHUNK_SIZE)
 		return xas_find_marked(xas, max, mark);
-	return xa_entry(xas->xa, node, offset);
+	entry = xa_entry(xas->xa, node, offset);
+	if (!entry)
+		return xas_find_marked(xas, max, mark);
+	return entry;
 }
 
 /*
diff --git a/lib/xarray.c b/lib/xarray.c
index f448bcd263ac..e9e641d3c0c3 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1208,6 +1208,8 @@ void *xas_find_marked(struct xa_state *xas, unsigned long max, xa_mark_t mark)
 		}
 
 		entry = xa_entry(xas->xa, xas->xa_node, xas->xa_offset);
+		if (!entry && !(xa_track_free(xas->xa) && mark == XA_FREE_MARK))
+			continue;
 		if (!xa_is_node(entry))
 			return entry;
 		xas->xa_node = xa_to_node(entry);
diff --git a/tools/testing/radix-tree/Makefile b/tools/testing/radix-tree/Makefile
index 397d6b612502..aa6abfe0749c 100644
--- a/tools/testing/radix-tree/Makefile
+++ b/tools/testing/radix-tree/Makefile
@@ -7,8 +7,8 @@ LDLIBS+= -lpthread -lurcu
 TARGETS = main idr-test multiorder xarray
 CORE_OFILES := xarray.o radix-tree.o idr.o linux.o test.o find_bit.o bitmap.o
 OFILES = main.o $(CORE_OFILES) regression1.o regression2.o regression3.o \
-	 regression4.o \
-	 tag_check.o multiorder.o idr-test.o iteration_check.o benchmark.o
+	 regression4.o tag_check.o multiorder.o idr-test.o iteration_check.o \
+	 iteration_check_2.o benchmark.o
 
 ifndef SHIFT
 	SHIFT=3
diff --git a/tools/testing/radix-tree/iteration_check_2.c b/tools/testing/radix-tree/iteration_check_2.c
new file mode 100644
index 000000000000..aac5c50a3674
--- /dev/null
+++ b/tools/testing/radix-tree/iteration_check_2.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * iteration_check_2.c: Check that deleting a tagged entry doesn't cause
+ * an RCU walker to finish early.
+ * Copyright (c) 2020 Oracle
+ * Author: Matthew Wilcox <willy@infradead.org>
+ */
+#include <pthread.h>
+#include "test.h"
+
+static volatile bool test_complete;
+
+static void *iterator(void *arg)
+{
+	XA_STATE(xas, arg, 0);
+	void *entry;
+
+	rcu_register_thread();
+
+	while (!test_complete) {
+		xas_set(&xas, 0);
+		rcu_read_lock();
+		xas_for_each_marked(&xas, entry, ULONG_MAX, XA_MARK_0)
+			;
+		rcu_read_unlock();
+		assert(xas.xa_index >= 100);
+	}
+
+	rcu_unregister_thread();
+	return NULL;
+}
+
+static void *throbber(void *arg)
+{
+	struct xarray *xa = arg;
+
+	rcu_register_thread();
+
+	while (!test_complete) {
+		int i;
+
+		for (i = 0; i < 100; i++) {
+			xa_store(xa, i, xa_mk_value(i), GFP_KERNEL);
+			xa_set_mark(xa, i, XA_MARK_0);
+		}
+		for (i = 0; i < 100; i++)
+			xa_erase(xa, i);
+	}
+
+	rcu_unregister_thread();
+	return NULL;
+}
+
+void iteration_test2(unsigned test_duration)
+{
+	pthread_t threads[2];
+	DEFINE_XARRAY(array);
+	int i;
+
+	printv(1, "Running iteration test 2 for %d seconds\n", test_duration);
+
+	test_complete = false;
+
+	xa_store(&array, 100, xa_mk_value(100), GFP_KERNEL);
+	xa_set_mark(&array, 100, XA_MARK_0);
+
+	if (pthread_create(&threads[0], NULL, iterator, &array)) {
+		perror("create iterator thread");
+		exit(1);
+	}
+	if (pthread_create(&threads[1], NULL, throbber, &array)) {
+		perror("create throbber thread");
+		exit(1);
+	}
+
+	sleep(test_duration);
+	test_complete = true;
+
+	for (i = 0; i < 2; i++) {
+		if (pthread_join(threads[i], NULL)) {
+			perror("pthread_join");
+			exit(1);
+		}
+	}
+
+	xa_destroy(&array);
+}
diff --git a/tools/testing/radix-tree/main.c b/tools/testing/radix-tree/main.c
index 7a22d6e3732e..f2cbc8e5b97c 100644
--- a/tools/testing/radix-tree/main.c
+++ b/tools/testing/radix-tree/main.c
@@ -311,6 +311,7 @@ int main(int argc, char **argv)
 	regression4_test();
 	iteration_test(0, 10 + 90 * long_run);
 	iteration_test(7, 10 + 90 * long_run);
+	iteration_test2(10 + 90 * long_run);
 	single_thread_tests(long_run);
 
 	/* Free any remaining preallocated nodes */
diff --git a/tools/testing/radix-tree/test.h b/tools/testing/radix-tree/test.h
index 1ee4b2c0ad10..34dab4d18744 100644
--- a/tools/testing/radix-tree/test.h
+++ b/tools/testing/radix-tree/test.h
@@ -34,6 +34,7 @@ void xarray_tests(void);
 void tag_check(void);
 void multiorder_checks(void);
 void iteration_test(unsigned order, unsigned duration);
+void iteration_test2(unsigned duration);
 void benchmark(void);
 void idr_checks(void);
 void ida_tests(void);
-- 
2.25.1