On Mon, Jul 06, 2015 at 08:49:33PM +0100, Waiman Long wrote: > On 07/06/2015 02:23 PM, Will Deacon wrote: > > I've just finished rebasing my arm64 qrwlock stuff, but I think it will > > conflict with these patches. Do you mind if I post them for review anyway, > > so we can at least co-ordinate our efforts? > > Yes, sure. I would also like to coordinate my changes with yours to > minimize conflict. BTW, I just got 2 tip-bot messages about the commits: > > locking/qrwlock: Better optimization for interrupt context readers > locking/qrwlock: Rename functions to queued_*() > > So I need to rebase my patches also. Yeah, I've been carrying those two on my branch as well, but everything should rebase cleanly. > >> --- > >> kernel/locking/qrwlock.c | 12 ++++-------- > >> 1 files changed, 4 insertions(+), 8 deletions(-) > >> > >> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c > >> index 81bae99..ecd2d19 100644 > >> --- a/kernel/locking/qrwlock.c > >> +++ b/kernel/locking/qrwlock.c > >> @@ -88,15 +88,11 @@ void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts) > >> arch_spin_lock(&lock->lock); > >> > >> /* > >> - * At the head of the wait queue now, wait until the writer state > >> - * goes to 0 and then try to increment the reader count and get > >> - * the lock. It is possible that an incoming writer may steal the > >> - * lock in the interim, so it is necessary to check the writer byte > >> - * to make sure that the write lock isn't taken. > >> + * At the head of the wait queue now, increment the reader count > >> + * and wait until the writer, if it has the lock, has gone away. > >> + * At ths stage, it is not possible for a writer to remain in the > >> + * waiting state (_QW_WAITING). So there won't be any deadlock. > >> */ > >> - while (atomic_read(&lock->cnts)& _QW_WMASK) > >> - cpu_relax_lowlatency(); > > Thinking about it, can we kill _QW_WAITING altogether and set (cmpxchg > > from 0) wmode to _QW_LOCKED in the write_lock slowpath, polling (acquire) > > rmode until it hits zero? > > No, this is how we make the lock fair so that an incoming streams of > later readers won't block a writer from getting the lock. But won't those readers effectively see that the lock is held for write (because we set wmode to _QW_LOCKED before the existing reader had drained) and therefore fall down the slow-path and get held up on the spinlock? 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
