bcache: avoid a deadlock in bcache_reboot()

It is quite frequently to observe deadlock in bcache_reboot() happens
and hang the system reboot process. The reason is, in bcache_reboot()
when calling bch_cache_set_stop() and bcache_device_stop() the mutex
bch_register_lock is held. But in the process to stop cache set and
bcache device, bch_register_lock will be acquired again. If this mutex
is held here, deadlock will happen inside the stopping process. The
aftermath of the deadlock is, whole system reboot gets hung.

The fix is to avoid holding bch_register_lock for the following loops
in bcache_reboot(),
       list_for_each_entry_safe(c, tc, &bch_cache_sets, list)
                bch_cache_set_stop(c);

        list_for_each_entry_safe(dc, tdc, &uncached_devices, list)
                bcache_device_stop(&dc->disk);

A module range variable 'bcache_is_reboot' is added, it sets to true
in bcache_reboot(). In register_bcache(), if bcache_is_reboot is checked
to be true, reject the registration by returning -EBUSY immediately.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 26 insertions, 0 deletions

diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index dddb8d4048ce..d62e28643109 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -16,6 +16,8 @@
 #include <linux/sort.h>
 #include <linux/sched/clock.h>
 
+extern bool bcache_is_reboot;
+
 /* Default is 0 ("writethrough") */
 static const char * const bch_cache_modes[] = {
 	"writethrough",
@@ -267,6 +269,10 @@ STORE(__cached_dev)
 	struct cache_set *c;
 	struct kobj_uevent_env *env;
 
+	/* no user space access if system is rebooting */
+	if (bcache_is_reboot)
+		return -EBUSY;
+
 #define d_strtoul(var)		sysfs_strtoul(var, dc->var)
 #define d_strtoul_nonzero(var)	sysfs_strtoul_clamp(var, dc->var, 1, INT_MAX)
 #define d_strtoi_h(var)		sysfs_hatoi(var, dc->var)
@@ -407,6 +413,10 @@ STORE(bch_cached_dev)
 	struct cached_dev *dc = container_of(kobj, struct cached_dev,
 					     disk.kobj);
 
+	/* no user space access if system is rebooting */
+	if (bcache_is_reboot)
+		return -EBUSY;
+
 	mutex_lock(&bch_register_lock);
 	size = __cached_dev_store(kobj, attr, buf, size);
 
@@ -510,6 +520,10 @@ STORE(__bch_flash_dev)
 					       kobj);
 	struct uuid_entry *u = &d->c->uuids[d->id];
 
+	/* no user space access if system is rebooting */
+	if (bcache_is_reboot)
+		return -EBUSY;
+
 	sysfs_strtoul(data_csum,	d->data_csum);
 
 	if (attr == &sysfs_size) {
@@ -745,6 +759,10 @@ STORE(__bch_cache_set)
 	struct cache_set *c = container_of(kobj, struct cache_set, kobj);
 	ssize_t v;
 
+	/* no user space access if system is rebooting */
+	if (bcache_is_reboot)
+		return -EBUSY;
+
 	if (attr == &sysfs_unregister)
 		bch_cache_set_unregister(c);
 
@@ -864,6 +882,10 @@ STORE(bch_cache_set_internal)
 {
 	struct cache_set *c = container_of(kobj, struct cache_set, internal);
 
+	/* no user space access if system is rebooting */
+	if (bcache_is_reboot)
+		return -EBUSY;
+
 	return bch_cache_set_store(&c->kobj, attr, buf, size);
 }
 
@@ -1049,6 +1071,10 @@ STORE(__bch_cache)
 	struct cache *ca = container_of(kobj, struct cache, kobj);
 	ssize_t v;
 
+	/* no user space access if system is rebooting */
+	if (bcache_is_reboot)
+		return -EBUSY;
+
 	if (attr == &sysfs_discard) {
 		bool v = strtoul_or_return(buf);