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Date: Sun, 22 May 2022 09:31:46 +0200
From: Donald Buczek <buczek@...gen.mpg.de>
To: Logan Gunthorpe <logang@...tatee.com>,
linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
Song Liu <song@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>,
Guoqing Jiang <guoqing.jiang@...ux.dev>,
Xiao Ni <xni@...hat.com>, Stephen Bates <sbates@...thlin.com>,
Martin Oliveira <Martin.Oliveira@...eticom.com>,
David Sloan <David.Sloan@...eticom.com>
Subject: Re: [PATCH v1 12/15] md/raid5-cache: Add RCU protection to conf->log
accesses
On 19.05.22 21:13, Logan Gunthorpe wrote:
> The mdadm test 21raid5cache randomly fails with NULL pointer accesses
> conf->log when run repeatedly. conf->log was sort of protected with
> a RCU, but most dereferences were not done with the correct functions.
>
> Add rcu_read_locks() and rcu_access_pointers() to the appropriate
> places.
>
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> ---
> drivers/md/raid5-cache.c | 135 +++++++++++++++++++++++++++------------
> drivers/md/raid5-log.h | 14 ++--
> drivers/md/raid5.c | 4 +-
> drivers/md/raid5.h | 2 +-
> 4 files changed, 104 insertions(+), 51 deletions(-)
>
> diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
> index f7b402138d16..1dbc7c4b9a15 100644
> --- a/drivers/md/raid5-cache.c
> +++ b/drivers/md/raid5-cache.c
> @@ -254,7 +254,14 @@ static bool __r5c_is_writeback(struct r5l_log *log)
>
> bool r5c_is_writeback(struct r5conf *conf)
> {
> - return __r5c_is_writeback(conf->log);
> + struct r5l_log *log;
> + bool ret;
> +
> + rcu_read_lock();
> + log = rcu_dereference(conf->log);
> + ret = __r5c_is_writeback(log);
> + rcu_read_unlock();
> + return ret;
> }
>
> static sector_t r5l_ring_add(struct r5l_log *log, sector_t start, sector_t inc)
> @@ -340,12 +347,23 @@ 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, 1);
> +}
> +
> /* 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))
> + if (!__r5c_is_writeback(log))
> return;
>
> total_cached = atomic_read(&conf->r5c_cached_partial_stripes) +
> @@ -361,7 +379,7 @@ 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);
> }
>
> /*
> @@ -370,8 +388,7 @@ void r5c_check_stripe_cache_usage(struct r5conf *conf)
> */
> void r5c_check_cached_full_stripe(struct r5conf *conf)
> {
> - if (!r5c_is_writeback(conf))
> - return;
> + struct r5l_log *log = get_log_for_io(conf);
>
> /*
> * wake up reclaim for R5C_FULL_STRIPE_FLUSH_BATCH cached stripes
> @@ -380,7 +397,7 @@ 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);
> }
>
> /*
> @@ -703,7 +720,7 @@ 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) {
> @@ -1001,7 +1018,7 @@ 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;
> @@ -1099,7 +1116,8 @@ 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);
> @@ -1109,7 +1127,7 @@ 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) {
> /*
> @@ -1297,7 +1315,7 @@ 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)
> @@ -1412,7 +1430,7 @@ 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;
> @@ -1431,9 +1449,8 @@ 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;
> @@ -1441,7 +1458,7 @@ static void r5c_do_reclaim(struct r5conf *conf)
> int stripes_to_flush;
> int flushing_partial, flushing_full;
>
> - if (!r5c_is_writeback(conf))
> + if (!__r5c_is_writeback(log))
> return;
>
> flushing_partial = atomic_read(&conf->r5c_flushing_partial_stripes);
> @@ -1561,22 +1578,30 @@ 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, 1);
> 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);
> + struct r5l_log *log = get_log_for_io(conf);
> +
> + __r5l_wake_reclaim(log, 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) {
> @@ -2534,16 +2559,20 @@ 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;
> +
> + rcu_read_lock();
> + log = rcu_dereference(conf->log);
> + if (!log)
> + goto out;
>
> - switch (conf->log->r5c_journal_mode) {
> + switch (log->r5c_journal_mode) {
> case R5C_JOURNAL_MODE_WRITE_THROUGH:
> ret = snprintf(
> page, PAGE_SIZE, "[%s] %s\n",
> @@ -2559,6 +2588,10 @@ 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;
> }
> @@ -2572,13 +2605,15 @@ 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 &&
> @@ -2586,12 +2621,19 @@ 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);
>
> @@ -2637,7 +2679,7 @@ 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;
> @@ -2804,7 +2846,7 @@ 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;
> @@ -2887,7 +2929,7 @@ 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;
> @@ -2943,7 +2985,7 @@ 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;
>
> @@ -2953,6 +2995,7 @@ 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;
> }
>
> @@ -3033,30 +3076,38 @@ 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)
> @@ -3164,15 +3215,17 @@ 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 f26e6f4c7f9a..24b4dbd5b25c 100644
> --- a/drivers/md/raid5-log.h
> +++ b/drivers/md/raid5-log.h
> @@ -58,7 +58,7 @@ 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)) {
A problem here is that `struct r5l_log` of `conf->log` is private to raid5-cache.c and gcc below version 10 (wrongly) regards the `typeof(*p) *local` declaration of __rcu_access_pointer as a dereference:
CC drivers/md/raid5.o
In file included from ./include/linux/rculist.h:11:0,
from ./include/linux/dcache.h:8,
from ./include/linux/fs.h:8,
from ./include/linux/highmem.h:5,
from ./include/linux/bvec.h:10,
from ./include/linux/blk_types.h:10,
from ./include/linux/blkdev.h:9,
from drivers/md/raid5.c:38:
drivers/md/raid5-log.h: In function ‘log_stripe’:
./include/linux/rcupdate.h:384:9: error: dereferencing pointer to incomplete type ‘struct r5l_log’
typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
^
./include/linux/rcupdate.h:495:31: note: in expansion of macro ‘__rcu_access_pointer’
#define rcu_access_pointer(p) __rcu_access_pointer((p), __UNIQUE_ID(rcu), __rcu)
^~~~~~~~~~~~~~~~~~~~
drivers/md/raid5-log.h:61:6: note: in expansion of macro ‘rcu_access_pointer’
if (rcu_access_pointer(conf->log)) {
^~~~~~~~~~~~~~~~~~
make[2]: *** [scripts/Makefile.build:288: drivers/md/raid5.o] Error 1
make[1]: *** [scripts/Makefile.build:550: drivers/md] Error 2
make: *** [Makefile:1834: drivers] Error 2
See https://godbolt.org/z/TPP8MdKbc to test compiler versions with this construct.
Best
Donald
> if (!test_bit(STRIPE_R5C_CACHING, &sh->state)) {
> /* writing out phase */
> if (s->waiting_extra_page)
> @@ -79,7 +79,7 @@ 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);
> @@ -87,7 +87,7 @@ 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);
> @@ -95,7 +95,7 @@ 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);
> @@ -105,7 +105,7 @@ 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);
> @@ -115,7 +115,7 @@ 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);
> @@ -123,7 +123,7 @@ 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 37fe2af77c93..c06c9ef88417 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;
--
Donald Buczek
buczek@...gen.mpg.de
Tel: +49 30 8413 1433
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