+
+ /* LU-8791: take oo_guard to avoid the deadlock that changing block
+ * size and assigning arcbuf take place at the same time.
+ *
+ * Thread 1:
+ * osd_write_commit()
+ * -> osd_grow_blocksize() with osd_object::oo_guard held
+ * -> dmu_object_set_blocksize()
+ * -> dnode_set_blksz(), with dnode_t::dn_struct_rwlock
+ * write lock held
+ * -> dbuf_new_size()
+ * -> dmu_buf_will_dirty()
+ * -> dbuf_read()
+ * -> wait for the dbuf state to change
+ * Thread 2:
+ * osd_write_commit()
+ * -> dmu_assign_arcbuf()
+ * -> dbuf_assign_arcbuf(), set dbuf state to DB_FILL
+ * -> dbuf_dirty()
+ * -> try to hold the read lock of dnode_t::dn_struct_rwlock
+ *
+ * By taking the read lock, it can avoid thread 2 to enter into the
+ * critical section of assigning the arcbuf, while thread 1 is
+ * changing the block size.
+ */
+ down_read(&obj->oo_guard);