[PATCH V6 10/10] null_blk: serialize configfs attribute shows with the lock

From: Zizhi Wo <[email protected]>

The _show callback in the NULLB_DEVICE_ATTR macro reads dev->NAME and the
_store path writes it. configfs does not serialize accesses across separate
open file descriptions (buffer->mutex is per-fd), and _show takes no lock,
so a concurrent read and write on the same attribute is a data race. The
_show readers also race against writes to these fields that run after the
configfs item becomes visible, e.g. in nullb_update_nr_hw_queues().

All of those writers now run under the file-scope lock: _store takes it
unconditionally, and the setup-side writers run under power_store() which
holds the same lock. The only remaining unsynchronized accesses are the
plain reads in _show. Rather than annotating every field with
READ_ONCE()/WRITE_ONCE() across files, simply take the file-scope lock in
_show (and in power_show) as well. This closes the remaining _show-vs-write
data races with a single lock and keeps the writers as plain assignments.

configfs attribute access is not on the I/O hot path, so taking the mutex
in _show is acceptable from a performance standpoint. The dev fields
written in null_alloc_dev() and dev->power in nullb_group_drop_item() need
no locking: the former runs from .make_group before the item is published,
and the latter is serialized by configfs frag_sem/frag_dead against
attribute show/store.

Suggested-by: Nilay Shroff <[email protected]>
Suggested-by: Bart Van Assche <[email protected]>
Signed-off-by: Zizhi Wo <[email protected]>
Reviewed-by: Nilay Shroff <[email protected]>
---
 drivers/block/null_blk/main.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index c7dfbec3e7d6..1c20d773dd0c 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -345,6 +345,7 @@ static ssize_t nullb_device_bool_attr_store(bool *val, const char *page,
 static ssize_t								\
 nullb_device_##NAME##_show(struct config_item *item, char *page)	\
 {									\
+	guard(mutex)(&lock);						\
 	return nullb_device_##TYPE##_attr_show(				\
 				to_nullb_device(item)->NAME, page);	\
 }									\
@@ -476,6 +477,7 @@ NULLB_DEVICE_ATTR(badblocks_partial_io, bool, NULL);
 
 static ssize_t nullb_device_power_show(struct config_item *item, char *page)
 {
+	guard(mutex)(&lock);
 	return nullb_device_bool_attr_show(to_nullb_device(item)->power, page);
 }
 
-- 
2.52.0