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    SubjectRe: [PATCH 5/6] locking/rwsem: Optimize slowpath/sleeping
    On Sun, Jan 25, 2015 at 11:36:08PM -0800, Davidlohr Bueso wrote:
    > When blocking , we incur in multiple barriers when setting the
    > task's uninterruptable state. This is particularly bad when the
    > lock keeps getting stolen from the task trying to acquire the sem.
    > These changes propose delaying setting the task's new state until
    > we are sure that calling schedule is inevitable.
    >
    > This implies that we do the trylock and active check (both basically
    > ->counter checks) as TASK_RUNNING. For the trylock we hold the wait
    > lock with interrupts disabled, so no risk there. And for the active
    > check, the window for which we could get interrupted is quite small
    > and makes no tangible difference.
    >
    > This patch increases Unixbench's 'execl' throughput by 25% on a 40
    > core machine.
    >
    > Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
    > ---
    > kernel/locking/rwsem-xadd.c | 20 +++++++++++++++-----
    > 1 file changed, 15 insertions(+), 5 deletions(-)
    >
    > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
    > index 18a50da..88b3468 100644
    > --- a/kernel/locking/rwsem-xadd.c
    > +++ b/kernel/locking/rwsem-xadd.c
    > @@ -459,17 +459,27 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
    > count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
    >
    > /* wait until we successfully acquire the lock */
    > while (true) {
    > if (rwsem_try_write_lock(count, sem))
    > break;
    > +
    > + __set_current_state(TASK_UNINTERRUPTIBLE);
    > raw_spin_unlock_irq(&sem->wait_lock);
    >
    > + /*
    > + * When there are active locks after we wake up,
    > + * the lock was probably stolen from us. Thus,
    > + * go immediately back to sleep and avoid taking
    > + * the wait_lock.
    > + */
    > + while (true) {
    > schedule();
    > +
    > + count = READ_ONCE(sem->count);
    > + if (!(count & RWSEM_ACTIVE_MASK))
    > + break;
    > + __set_current_state(TASK_UNINTERRUPTIBLE);
    > + }

    So its late and I'm not seeing it; why is this safe? How will we not
    miss the wakeup that makes condition true?

    >
    > raw_spin_lock_irq(&sem->wait_lock);
    > }


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