Messages in this thread | | | Date | Wed, 11 May 2016 20:38:09 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/rwsem: Optimize write lock slowpath |
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On Wed, May 11, 2016 at 11:26:02AM -0700, Jason Low wrote: > On Wed, 2016-05-11 at 13:49 +0200, Peter Zijlstra wrote:
> > > static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) > > > { > > > /* > > > + * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS. > > > */ > > > + if (count != RWSEM_WAITING_BIAS) > > > + return false; > > > + > > > + /* > > > + * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there > > > + * are other tasks on the wait list, we need to add on WAITING_BIAS. > > > + */ > > > + count = list_is_singular(&sem->wait_list) ? > > > + RWSEM_ACTIVE_WRITE_BIAS : > > > + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS; > > > + > > > + if (cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) == RWSEM_WAITING_BIAS) { > > > rwsem_set_owner(sem); > > > return true; > > > } > > > > Right; so that whole thing works because we're holding sem->wait_lock. > > Should we clarify that someplace? > > Yup, we can mention that the rwsem_try_write_lock() function must be > called with the wait_lock held.
Also try to explain _why_ it must be held.
Thanks!
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