lkml.org 
[lkml]   [2014]   [Apr]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v2] rwsem: Support optimistic spinning
    On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
    > +/*
    > + * Try to acquire write lock before the writer has been put on wait queue.
    > + */
    > +static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
    > +{
    > + long count = ACCESS_ONCE(sem->count);
    > +retry:
    > + if (count == RWSEM_WAITING_BIAS) {
    > + count = cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
    > + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS);
    > + /* allow write lock stealing, try acquiring the write lock. */
    > + if (count == RWSEM_WAITING_BIAS)
    > + goto acquired;
    > + else if (count == 0)
    > + goto retry;
    > + } else if (count == 0) {
    > + count = cmpxchg(&sem->count, 0, RWSEM_ACTIVE_WRITE_BIAS);
    > + if (count == 0)
    > + goto acquired;
    > + else if (count == RWSEM_WAITING_BIAS)
    > + goto retry;
    > + }
    > + return false;
    > +
    > +acquired:
    > + return true;
    > +}

    Could we have written that like:

    static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
    {
    long old, count = ACCESS_ONCE(sem->count);

    for (;;) {
    if (!(count == 0 || count == RWSEM_WAITING_BIAS))
    return false;

    old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS);
    if (old == count)
    return true;

    count = old;
    }
    }

    ?

    Just checking my brain got all the branches untangled right :-)


    \
     
     \ /
      Last update: 2014-04-30 11:01    [W:2.758 / U:0.168 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site