lkml.org 
[lkml]   [2016]   [May]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] locking/rwsem: Optimize write lock slowpath
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!

\
 
 \ /
  Last update: 2016-05-11 21:01    [W:0.081 / U:0.028 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site