Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE

From: Waiman Long
Date: Tue May 24 2016 - 21:26:02 EST


On 05/23/2016 02:46 PM, Jason Low wrote:
On Sat, 2016-05-21 at 09:04 -0700, Peter Hurley wrote:
On 05/18/2016 12:58 PM, Jason Low wrote:
It should be fine to use the standard READ_ONCE here, even if it's just
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.
See, I don't understand this line of reasoning at all.

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".
So I think there is a difference between using it during init time and
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_.
What about reading ->on_cpu locklessly?
Sure, we could also use READ_ONCE when reading ->on_cpu :)


As on_cpu is just a boolean, load tearing isn't really a problem. You either see the bit 0 set or not, but not something in between (not a qbit) :-)

Cheers,
Longman