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 05/15, Peter Zijlstra wrote:
>
> > > 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.

Yep. So rwsem_spin_on_owner() goes to "out:" and returns
!rwsem_owner_is_reader() == T.

IOW, afaics owner == NULL means "spin unconditionally", I guess this is for
the case when the new writer is going to do rwsem_set_owner() or up_write()
has already called rwsem_clear_owner() but didn't do up_write() yet.

Probably makes sense, but the code is not very clean,

> Or am I completely confused again?

Or me, I am not sure.

Oleg.

\
 
 \ /
  Last update: 2018-05-15 14:46    [W:0.083 / U:0.172 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site