[PATCH v2 13/17] md/raid5-cache: Add RCU protection to conf->log accesses

Logan Gunthorpe
2022-05-26T16:37:39Z

The mdadm test 21raid5cache randomly fails with NULL pointer accesses
of conf->log when run repeatedly. conf->log was sort of protected with
RCU, but most dereferences were not done with the correct functions.

Add rcu_read_locks(), rcu_dereference_protected() and rcu_access_pointers()
calls to the appropriate places and mark the pointer with __rcu.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
---
drivers/md/raid5-cache.c | 132 +++++++++++++++++++++++++++------------
drivers/md/raid5-log.h | 14 ++---
drivers/md/raid5.c | 4 +-
drivers/md/raid5.h | 2 +-
4 files changed, 102 insertions(+), 50 deletions(-)

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 6349cfaae2c8..7c56ee977c3a 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -180,7 +180,12 @@ static bool r5c_is_writeback(struct r5l_log *log)

bool r5c_conf_is_writeback(struct r5conf *conf)
{
- return r5c_is_writeback(conf->log);
+ bool ret;
+
+ rcu_read_lock();
+ ret = r5c_is_writeback(rcu_dereference(conf->log));
+ rcu_read_unlock();
+ return ret;
}

static sector_t r5l_ring_add(struct r5l_log *log, sector_t start, sector_t inc)
@@ -266,12 +271,23 @@ static void __r5l_wake_reclaim(struct r5l_log *log, sector_t space)
md_wakeup_thread(log->reclaim_thread);
}

+static struct r5l_log *get_log_for_io(struct r5conf *conf)
+{
+ /*
+ * rcu_dereference_protected is safe because the array will be
+ * quiesced before log_exit() so it can't be called while
+ * an IO is in progress.
+ */
+ return rcu_dereference_protected(conf->log, true);
+}
+
/* Check whether we should flush some stripes to free up stripe cache */
void r5c_check_stripe_cache_usage(struct r5conf *conf)
{
+ struct r5l_log *log = get_log_for_io(conf);
int total_cached;

- if (!r5c_is_writeback(conf->log))
+ if (!r5c_is_writeback(log))
return;

total_cached = atomic_read(&conf->r5c_cached_partial_stripes) +
@@ -287,7 +303,7 @@ void r5c_check_stripe_cache_usage(struct r5conf *conf)
*/
if (total_cached > conf->min_nr_stripes * 1 / 2 ||
atomic_read(&conf->empty_inactive_list_nr) > 0)
- __r5l_wake_reclaim(conf->log, 0);
+ __r5l_wake_reclaim(log, 0);
}

/*
@@ -296,7 +312,9 @@ void r5c_check_stripe_cache_usage(struct r5conf *conf)
*/
void r5c_check_cached_full_stripe(struct r5conf *conf)
{
- if (!r5c_is_writeback(conf->log))
+ struct r5l_log *log = get_log_for_io(conf);
+
+ if (!r5c_is_writeback(log))
return;

/*
@@ -306,7 +324,7 @@ void r5c_check_cached_full_stripe(struct r5conf *conf)
if (atomic_read(&conf->r5c_cached_full_stripes) >=
min(R5C_FULL_STRIPE_FLUSH_BATCH(conf),
conf->chunk_sectors >> RAID5_STRIPE_SHIFT(conf)))
- __r5l_wake_reclaim(conf->log, 0);
+ __r5l_wake_reclaim(log, 0);
}

/*
@@ -388,7 +406,7 @@ static inline void r5c_update_log_state(struct r5l_log *log)
void r5c_make_stripe_write_out(struct stripe_head *sh)
{
struct r5conf *conf = sh->raid_conf;
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);

BUG_ON(!r5c_is_writeback(log));

@@ -630,7 +648,7 @@ static void r5c_disable_writeback_async(struct work_struct *work)

/* wait superblock change before suspend */
wait_event(mddev->sb_wait,
- conf->log == NULL ||
+ !rcu_access_pointer(conf->log) ||
(!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags) &&
(locked = mddev_trylock(mddev))));
if (locked) {
@@ -928,7 +946,7 @@ static inline void r5l_add_no_space_stripe(struct r5l_log *log,
*/
int r5l_write_stripe(struct r5conf *conf, struct stripe_head *sh)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);
int write_disks = 0;
int data_pages, parity_pages;
int reserve;
@@ -1026,7 +1044,8 @@ int r5l_write_stripe(struct r5conf *conf, struct stripe_head *sh)

void r5l_write_stripe_run(struct r5conf *conf)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);
+
if (!log)
return;
mutex_lock(&log->io_mutex);
@@ -1036,7 +1055,7 @@ void r5l_write_stripe_run(struct r5conf *conf)

int r5l_handle_flush_request(struct r5conf *conf, struct bio *bio)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);

if (log->r5c_journal_mode == R5C_JOURNAL_MODE_WRITE_THROUGH) {
/*
@@ -1224,7 +1243,7 @@ static void r5l_log_flush_endio(struct bio *bio)
*/
void r5l_flush_stripe_to_raid(struct r5conf *conf)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);
bool do_flush;

if (!log || !log->need_cache_flush)
@@ -1339,7 +1358,7 @@ void r5c_flush_cache(struct r5conf *conf, int num)
struct stripe_head *sh, *next;

lockdep_assert_held(&conf->device_lock);
- if (!conf->log)
+ if (!rcu_access_pointer(conf->log))
return;

count = 0;
@@ -1358,9 +1377,8 @@ void r5c_flush_cache(struct r5conf *conf, int num)
}
}

-static void r5c_do_reclaim(struct r5conf *conf)
+static void r5c_do_reclaim(struct r5conf *conf, struct r5l_log *log)
{
- struct r5l_log *log = conf->log;
struct stripe_head *sh;
int count = 0;
unsigned long flags;
@@ -1488,22 +1506,28 @@ static void r5l_reclaim_thread(struct md_thread *thread)
{
struct mddev *mddev = thread->mddev;
struct r5conf *conf = mddev->private;
- struct r5l_log *log = conf->log;
+ struct r5l_log *log;

+ /*
+ * This is safe, because the reclaim thread will be stopped before
+ * the log is freed in log_exit()
+ */
+ log = rcu_dereference_protected(conf->log, true);
if (!log)
return;
- r5c_do_reclaim(conf);
+
+ r5c_do_reclaim(conf, log);
r5l_do_reclaim(log);
}

void r5l_wake_reclaim(struct r5conf *conf, sector_t space)
{
- __r5l_wake_reclaim(conf->log, space);
+ __r5l_wake_reclaim(get_log_for_io(conf), space);
}

void r5l_quiesce(struct r5conf *conf, int quiesce)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);
struct mddev *mddev;

if (quiesce) {
@@ -2461,16 +2485,20 @@ static void r5l_write_super(struct r5l_log *log, sector_t cp)
static ssize_t r5c_journal_mode_show(struct mddev *mddev, char *page)
{
struct r5conf *conf;
- int ret;
+ struct r5l_log *log;
+ int ret = 0;

spin_lock(&mddev->lock);
conf = mddev->private;
- if (!conf || !conf->log) {
- spin_unlock(&mddev->lock);
- return 0;
- }
+ if (!conf)
+ goto out_spin_unlock;

- switch (conf->log->r5c_journal_mode) {
+ rcu_read_lock();
+ log = rcu_dereference(conf->log);
+ if (!log)
+ goto out;
+
+ switch (log->r5c_journal_mode) {
case R5C_JOURNAL_MODE_WRITE_THROUGH:
ret = snprintf(
page, PAGE_SIZE, "[%s] %s\n",
@@ -2486,6 +2514,10 @@ static ssize_t r5c_journal_mode_show(struct mddev *mddev, char *page)
default:
ret = 0;
}
+
+out:
+ rcu_read_unlock();
+out_spin_unlock:
spin_unlock(&mddev->lock);
return ret;
}
@@ -2499,13 +2531,15 @@ static ssize_t r5c_journal_mode_show(struct mddev *mddev, char *page)
int r5c_journal_mode_set(struct mddev *mddev, int mode)
{
struct r5conf *conf;
+ struct r5l_log *log;
+ int ret = 0;

if (mode < R5C_JOURNAL_MODE_WRITE_THROUGH ||
mode > R5C_JOURNAL_MODE_WRITE_BACK)
return -EINVAL;

conf = mddev->private;
- if (!conf || !conf->log)
+ if (!conf || !rcu_access_pointer(conf->log))
return -ENODEV;

if (raid5_calc_degraded(conf) > 0 &&
@@ -2513,12 +2547,19 @@ int r5c_journal_mode_set(struct mddev *mddev, int mode)
return -EINVAL;

mddev_suspend(mddev);
- conf->log->r5c_journal_mode = mode;
+ rcu_read_lock();
+ log = rcu_dereference(conf->log);
+ if (log) {
+ log->r5c_journal_mode = mode;
+ pr_debug("md/raid:%s: setting r5c cache mode to %d: %s\n",
+ mdname(mddev), mode, r5c_journal_mode_str[mode]);
+ } else {
+ ret = -ENODEV;
+ }
+ rcu_read_unlock();
mddev_resume(mddev);

- pr_debug("md/raid:%s: setting r5c cache mode to %d: %s\n",
- mdname(mddev), mode, r5c_journal_mode_str[mode]);
- return 0;
+ return ret;
}
EXPORT_SYMBOL(r5c_journal_mode_set);

@@ -2564,7 +2605,7 @@ int r5c_try_caching_write(struct r5conf *conf,
struct stripe_head_state *s,
int disks)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);
int i;
struct r5dev *dev;
int to_cache = 0;
@@ -2731,7 +2772,7 @@ void r5c_finish_stripe_write_out(struct r5conf *conf,
struct stripe_head *sh,
struct stripe_head_state *s)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);
int i;
int do_wakeup = 0;
sector_t tree_index;
@@ -2814,7 +2855,7 @@ void r5c_finish_stripe_write_out(struct r5conf *conf,

int r5c_cache_data(struct r5conf *conf, struct stripe_head *sh)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);
int pages = 0;
int reserve;
int i;
@@ -2870,7 +2911,7 @@ int r5c_cache_data(struct r5conf *conf, struct stripe_head *sh)
/* check whether this big stripe is in write back cache. */
bool r5c_big_stripe_cached(struct r5conf *conf, sector_t sect)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log = get_log_for_io(conf);
sector_t tree_index;
void *slot;

@@ -2880,6 +2921,7 @@ bool r5c_big_stripe_cached(struct r5conf *conf, sector_t sect)
WARN_ON_ONCE(!rcu_read_lock_held());
tree_index = r5c_tree_index(conf, sect);
slot = radix_tree_lookup(&log->big_stripe_tree, tree_index);
+
return slot != NULL;
}

@@ -2960,30 +3002,38 @@ static int r5l_load_log(struct r5l_log *log)

int r5l_start(struct r5conf *conf)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log;
int ret;

+ log = rcu_dereference_protected(conf->log,
+ lockdep_is_held(&conf->mddev->reconfig_mutex));
if (!log)
return 0;

ret = r5l_load_log(log);
if (ret)
r5l_exit_log(conf);
+
return ret;
}

void r5c_update_on_rdev_error(struct mddev *mddev, struct md_rdev *rdev)
{
struct r5conf *conf = mddev->private;
- struct r5l_log *log = conf->log;
+ struct r5l_log *log;

+ rcu_read_lock();
+ log = rcu_dereference(conf->log);
if (!log)
- return;
+ goto out;

if ((raid5_calc_degraded(conf) > 0 ||
test_bit(Journal, &rdev->flags)) &&
- conf->log->r5c_journal_mode == R5C_JOURNAL_MODE_WRITE_BACK)
+ log->r5c_journal_mode == R5C_JOURNAL_MODE_WRITE_BACK)
schedule_work(&log->disable_writeback_work);
+
+out:
+ rcu_read_unlock();
}

int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)
@@ -3091,15 +3141,17 @@ int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)

void r5l_exit_log(struct r5conf *conf)
{
- struct r5l_log *log = conf->log;
+ struct r5l_log *log;

- conf->log = NULL;
- synchronize_rcu();
+ lockdep_assert_held(&conf->mddev->reconfig_mutex);
+ log = rcu_replace_pointer(conf->log, NULL, 1);

/* Ensure disable_writeback_work wakes up and exits */
wake_up(&conf->mddev->sb_wait);
flush_work(&log->disable_writeback_work);
md_unregister_thread(&log->reclaim_thread);
+ synchronize_rcu();
+
mempool_exit(&log->meta_pool);
bioset_exit(&log->bs);
mempool_exit(&log->io_pool);
diff --git a/drivers/md/raid5-log.h b/drivers/md/raid5-log.h
index 5948c41f1f2e..5b910e2d4279 100644
--- a/drivers/md/raid5-log.h
+++ b/drivers/md/raid5-log.h
@@ -133,7 +133,7 @@ static inline int log_stripe(struct stripe_head *sh, struct stripe_head_state *s
{
struct r5conf *conf = sh->raid_conf;

- if (conf->log) {
+ if (rcu_access_pointer(conf->log)) {
if (!test_bit(STRIPE_R5C_CACHING, &sh->state)) {
/* writing out phase */
if (s->waiting_extra_page)
@@ -154,7 +154,7 @@ static inline void log_stripe_write_finished(struct stripe_head *sh)
{
struct r5conf *conf = sh->raid_conf;

- if (conf->log)
+ if (rcu_access_pointer(conf->log))
r5l_stripe_write_finished(sh);
else if (raid5_has_ppl(conf))
ppl_stripe_write_finished(sh);
@@ -162,7 +162,7 @@ static inline void log_stripe_write_finished(struct stripe_head *sh)

static inline void log_write_stripe_run(struct r5conf *conf)
{
- if (conf->log)
+ if (rcu_access_pointer(conf->log))
r5l_write_stripe_run(conf);
else if (raid5_has_ppl(conf))
ppl_write_stripe_run(conf);
@@ -170,7 +170,7 @@ static inline void log_write_stripe_run(struct r5conf *conf)

static inline void log_flush_stripe_to_raid(struct r5conf *conf)
{
- if (conf->log)
+ if (rcu_access_pointer(conf->log))
r5l_flush_stripe_to_raid(conf);
else if (raid5_has_ppl(conf))
ppl_write_stripe_run(conf);
@@ -180,7 +180,7 @@ static inline int log_handle_flush_request(struct r5conf *conf, struct bio *bio)
{
int ret = -ENODEV;

- if (conf->log)
+ if (rcu_access_pointer(conf->log))
ret = r5l_handle_flush_request(conf, bio);
else if (raid5_has_ppl(conf))
ret = ppl_handle_flush_request(bio);
@@ -190,7 +190,7 @@ static inline int log_handle_flush_request(struct r5conf *conf, struct bio *bio)

static inline void log_quiesce(struct r5conf *conf, int quiesce)
{
- if (conf->log)
+ if (rcu_access_pointer(conf->log))
r5l_quiesce(conf, quiesce);
else if (raid5_has_ppl(conf))
ppl_quiesce(conf, quiesce);
@@ -198,7 +198,7 @@ static inline void log_quiesce(struct r5conf *conf, int quiesce)

static inline void log_exit(struct r5conf *conf)
{
- if (conf->log)
+ if (rcu_access_pointer(conf->log))
r5l_exit_log(conf);
else if (raid5_has_ppl(conf))
ppl_exit_log(conf);
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 4b25a0262cb5..93b4b69650bb 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -7937,7 +7937,7 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
struct md_rdev *tmp;

print_raid5_conf(conf);
- if (test_bit(Journal, &rdev->flags) && conf->log) {
+ if (test_bit(Journal, &rdev->flags) && rcu_access_pointer(conf->log)) {
mddev_suspend(mddev);
log_exit(conf);
mddev_resume(mddev);
@@ -8019,7 +8019,7 @@ static int raid5_add_disk(struct mddev *mddev, struct md_rdev *rdev)
int last = conf->raid_disks - 1;

if (test_bit(Journal, &rdev->flags)) {
- if (conf->log)
+ if (rcu_access_pointer(conf->log))
return -EBUSY;

rdev->raid_disk = 0;
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 638d29863503..04fe5b6f679c 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -684,7 +684,7 @@ struct r5conf {
struct r5worker_group *worker_groups;
int group_cnt;
int worker_cnt_per_group;
- struct r5l_log *log;
+ struct r5l_log __rcu *log;
void *log_private;

spinlock_t pending_bios_lock;
--
2.30.2