[Problem] A data race in snd_ctl_elem_add()

["Gong, Sishuai" <sishuai@purdue.edu>]

Hi,

We found a data race in sound/core/control.c in linux-5.12-rc3 and we are able to reproduce it under x86. 
In general, we found when 2 kernel threads are both running snd_ctl_elem_add(),
one may read a stale value of card->user_ctl_count, as shown below.

Currently, we haven’t found any explicit errors due to this data race, but it looks the reader threads 
may operate in an inconsistent  state, where card->user_ctl_count + 1 is actually bigger 
than MAX_USER_CONTROLS, so we want to point it out.
 
Thread 1					Thread 2
//snd_ctl_elem_add()		//snd_ctl_elem_add()
						if (card->user_ctl_count + 1 > MAX_USER_CONTROLS)
							return -ENOMEM;
						
card->user_ctl_count++;
unlock:
up_write(&card->controls_rwsem);
return err;


Thanks,
Sishuai