Messages in this thread | | | Date | Wed, 30 Apr 2014 10:27:15 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] rwsem: Support optimistic spinning |
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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 :-)
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