On Sun, Apr 25, 2021 at 06:50:48PM -0700, Bart Van Assche wrote:

On 4/25/21 5:49 PM, Ming Lei wrote:

quoted

On Sun, Apr 25, 2021 at 01:42:59PM -0700, Bart Van Assche wrote:

quoted

Using cmpxchg() on set->tags[] is only safe if all other set->tags[]
accesses are changed into WRITE_ONCE() or READ_ONCE().

Why?

Semantic of cmpxchg() is to modify value pointed by the address if its
old value is same with passed 'rq'. That is exactly what we need.

writting 'void *' is always atomic. if someone has touched
'->rqs[tag]', cmpxchg() won't modify the value.

WRITE_ONCE() supports data types that have the same size as char, short,
int, long and long long. That includes void *. If writes to these data
types would always be atomic then we wouldn't need the WRITE_ONCE()
macro.

OK, then we don't need WRITE_ONCE(), since WRITE on tags->rqs[i] is
always atomic.


Thanks, 
Ming