lkml.org 
[lkml]   [2014]   [Apr]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH v2] rwsem: Support optimistic spinning
    From
    Date
    On Wed, 2014-04-30 at 10:27 +0200, Peter Zijlstra wrote:
    > 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;
    > }
    > }
    >
    > ?

    Yeah that's a lot nicer.



    \
     
     \ /
      Last update: 2014-04-30 19:01    [W:4.140 / U:0.036 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site