lkml.org 
[lkml]   [2018]   [May]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()
On Tue, May 15, 2018 at 01:06:33PM +0200, Oleg Nesterov wrote:
> On 05/15, Peter Zijlstra wrote:
> >
> > So what's wrong with adding:
> >
> > if (!read)
> > sem->rw_sem.owner = current;
>
> Agreed, I have already suggested this change twice. Except we obviously
> need to check CONFIG_RWSEM_SPIN_ON_OWNER (->owner doesn't exists otherwise)
> or even CONFIG_DEBUG_RWSEMS to make the purpose more clear.

Right, details ;-)

> > Afaict the whole .owner=NULL thing in release already stops the spinners
>
> Not really, the new writer will spin in this case, afaics.
>
> But this is another problem and probably we do not care. The new writer is
> almost impossible in this particular case, another freeze_super() should
> notice frozen != SB_UNFROZEN and return EBUSY.

rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
&& owner != RWSEM_READER_OWNED, which will fail for !owner.

Or am I completely confused again?

> > and the above 'fixes' the debug splat.
>
> Yes.
>
> Waiman, can't we trivially fix the problem first? Then we can add the helpers
> and think about other improvements.

It is really simple; we're not going to add public (and EXPORT'ed to
boot) interfaces to rwsem for this.

\
 
 \ /
  Last update: 2018-05-15 13:52    [W:0.095 / U:4.148 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site