Re: [PATCH v2 5/7] locking/qrwlock: make use of acquire/release/relaxed atomics

Peter Zijlstra
2015-07-16T16:59:10Z

On Thu, Jul 16, 2015 at 04:32:36PM +0100, Will Deacon wrote:
> @@ -130,8 +130,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
> /*
> * Atomically decrement the reader count
> */
> - smp_mb__before_atomic();
> - atomic_sub(_QR_BIAS, &lock->cnts);
> + (void)atomic_sub_return_release(_QR_BIAS, &lock->cnts);
> }
>
> /**

This one will actually cause different code on x86; I think its still
fine though. LOCK XADD should not be (much) slower than LOCK SUB.

> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> index a71bb3541880..879c8fab7bea 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -36,7 +36,7 @@ rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
> {
> while ((cnts & _QW_WMASK) == _QW_LOCKED) {
> cpu_relax_lowlatency();
> - cnts = smp_load_acquire((u32 *)&lock->cnts);
> + cnts = atomic_read_acquire(&lock->cnts);
> }
> }

It might make sense to add comments to the users of this function that
actually rely on the _acquire semantics, I had to double check that :-)

But otherwise that all looks good.