On Fri, 2013-03-15 at 03:54 -0700, Michel Lespinasse wrote: > Using rwsem_atomic_update to try stealing the write lock forced us to > undo the adjustment in the failure path. We can have simpler and faster > code by using cmpxchg instead. > > Signed-off-by: Michel Lespinasse <[email protected]> > > --- > lib/rwsem.c | 26 ++++++-------------------- > 1 file changed, 6 insertions(+), 20 deletions(-) > > diff --git a/lib/rwsem.c b/lib/rwsem.c > index 2360bf204098..64c2dc007be2 100644 > --- a/lib/rwsem.c > +++ b/lib/rwsem.c > @@ -142,25 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) > return sem; > } > > -/* Try to get write sem, caller holds sem->wait_lock: */ > -static int try_get_writer_sem(struct rw_semaphore *sem) > -{ > - long oldcount, adjustment; > - > - adjustment = RWSEM_ACTIVE_WRITE_BIAS; > - if (list_is_singular(&sem->wait_list)) > - adjustment -= RWSEM_WAITING_BIAS; > - > -try_again_write: > - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; > - if (!(oldcount & RWSEM_ACTIVE_MASK)) > - return 1; > - /* some one grabbed the sem already */ > - if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) > - return 0; > - goto try_again_write; > -} > - > /* > * wait for the read lock to be granted > */ > @@ -236,7 +217,12 @@ struct rw_semaphore __sched > *rwsem_down_write_failed(struct rw_semaphore *sem) > while (true) { > set_task_state(tsk, TASK_UNINTERRUPTIBLE); > > - if (try_get_writer_sem(sem)) > + /* Try acquiring the write lock. */ > + count = RWSEM_ACTIVE_WRITE_BIAS; > + if (!list_is_singular(&sem->wait_list)) > + count += RWSEM_WAITING_BIAS; > + if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) == > + RWSEM_WAITING_BIAS)
This is just my opinion but I think this is way more readable if this stays in try_get_writer_sem() and you change that to static inline rather than inlining here. Especially because 9/13 adds a scope. > break; > > raw_spin_unlock_irq(&sem->wait_lock); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
