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    Subject[PATCH v2 04/13] rwsem: simplify rwsem_down_read_failed
    Date
    When trying to acquire a read lock, the RWSEM_ACTIVE_READ_BIAS adjustment
    doesn't cause other readers to block, so we never have to worry about waking
    them back after canceling this adjustment in rwsem_down_read_failed().

    We also never want to steal the lock in rwsem_down_read_failed(), so we
    don't have to grab the wait_lock either.

    Signed-off-by: Michel Lespinasse <walken@google.com>

    ---
    lib/rwsem.c | 22 ++--------------------
    1 file changed, 2 insertions(+), 20 deletions(-)

    diff --git a/lib/rwsem.c b/lib/rwsem.c
    index fb658af1c12c..66f307e90761 100644
    --- a/lib/rwsem.c
    +++ b/lib/rwsem.c
    @@ -182,7 +182,6 @@ try_again_write:
    */
    struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
    {
    - enum rwsem_waiter_type type = RWSEM_WAITING_FOR_READ;
    signed long adjustment = -RWSEM_ACTIVE_READ_BIAS;
    struct rwsem_waiter waiter;
    struct task_struct *tsk = current;
    @@ -190,7 +189,7 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)

    /* set up my own style of waitqueue */
    waiter.task = tsk;
    - waiter.type = type;
    + waiter.type = RWSEM_WAITING_FOR_READ;
    get_task_struct(tsk);

    raw_spin_lock_irq(&sem->wait_lock);
    @@ -201,17 +200,9 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
    /* we're now waiting on the lock, but no longer actively locking */
    count = rwsem_atomic_update(adjustment, sem);

    - /* If there are no active locks, wake the front queued process(es) up.
    - *
    - * Alternatively, if we're called from a failed down_write(), there
    - * were already threads queued before us and there are no active
    - * writers, the lock must be read owned; so we try to wake any read
    - * locks that were queued ahead of us. */
    + /* If there are no active locks, wake the front queued process(es). */
    if (count == RWSEM_WAITING_BIAS)
    sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
    - else if (count > RWSEM_WAITING_BIAS &&
    - adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
    - sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);

    raw_spin_unlock_irq(&sem->wait_lock);

    @@ -220,15 +211,6 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
    set_task_state(tsk, TASK_UNINTERRUPTIBLE);
    if (!waiter.task)
    break;
    -
    - raw_spin_lock_irq(&sem->wait_lock);
    - /* Try to get the writer sem, may steal from the head writer: */
    - if (type == RWSEM_WAITING_FOR_WRITE)
    - if (try_get_writer_sem(sem, &waiter)) {
    - raw_spin_unlock_irq(&sem->wait_lock);
    - return sem;
    - }
    - raw_spin_unlock_irq(&sem->wait_lock);
    schedule();
    }

    --
    1.8.1.3

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