On Tue, Jul 07, 2015 at 06:51:54PM +0100, Waiman Long wrote: > On 07/07/2015 01:24 PM, Will Deacon wrote: > > When a slow-path reader gets to the front of the wait queue outside of > > interrupt context, it waits for any writers to drain, increments the > > reader count and again waits for any additional writers that may have > > snuck in between the initial check and the increment. > > > > Given that this second check is performed with acquire semantics, there > > is no need to perform the increment using atomic_add_return, which acts > > as a full barrier. > > > > This patch changes the slow-path code to use smp_load_acquire and > > atomic_add instead of atomic_add_return. Since the check only involves > > the writer count, we can perform the acquire after the add. > > > > Signed-off-by: Will Deacon<will.deacon@xxxxxxx> > > --- > > kernel/locking/qrwlock.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c > > index 96b77d1e0545..4e29bef688ac 100644 > > --- a/kernel/locking/qrwlock.c > > +++ b/kernel/locking/qrwlock.c > > @@ -98,7 +98,8 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts) > > while (atomic_read(&lock->cnts)& _QW_WMASK) > > cpu_relax_lowlatency(); > > > > - cnts = atomic_add_return(_QR_BIAS,&lock->cnts) - _QR_BIAS; > > + atomic_add(_QR_BIAS,&lock->cnts); > > + cnts = smp_load_acquire((u32 *)&lock->cnts); > > rspin_until_writer_unlock(lock, cnts); > > > > /* > > Atomic add in x86 is actually a full barrier too. The performance > difference between "lock add" and "lock xadd" should be minor. The > additional load, however, could potentially cause an additional > cacheline load on a contended lock. So do you see actual performance > benefit of this change in ARM? I'd need to re-run the numbers, but atomic_add is significantly less work on ARM than atomic_add_return, which basically has two full memory barriers compared to none for the former. Will -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
