On Sat, 2016-05-21 at 09:04 -0700, Peter Hurley wrote:
On 05/18/2016 12:58 PM, Jason Low wrote:So I think there is a difference between using it during init time and
It should be fine to use the standard READ_ONCE here, even if it's justSee, I don't understand this line of reasoning at all.
for documentation, as it's probably not going to cost anything in
practice. It would be better to avoid adding any special macros for this
which may just add more complexity.
I read this as "it's ok to be non-optimal here where were spinning CPU
time but not ok to be non-optimal generally elsewhere where it's
way less important like at init time".
using it here where we're spinning. During init time, initializing the
owner field locklessly is normal. No other thread should be concurrently
be writing to the field, since the structure is just getting
initialized, so there are no surprises there.
Our access of the owner field in this function is special in that we're
using a bit of "lockless magic" to read and write to a field that gets
concurrently accessed without any serialization. Since we're not taking
the wait_lock in a scenario where we'd normally would take a lock, it
would be good to have this documented.
And by the way, it's not just "here" but _everywhere_.Sure, we could also use READ_ONCE when reading ->on_cpu :)
What about reading ->on_cpu locklessly?