lkml.org 
[lkml]   [2014]   [Dec]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] locking/rwlocks: clean up of qrwlock
On 12/16/14 at 10:01am, Peter Zijlstra wrote:
> On Tue, Dec 16, 2014 at 02:00:40PM +0800, Baoquan He wrote:
> > In queue_read_lock_slowpath, when writer count becomes 0, we need
> > increment the read count and get the lock. Then need call
> > rspin_until_writer_unlock to check again if an incoming writer
> > steals the lock in the gap. But in rspin_until_writer_unlock
> > it only checks the writer count, namely low 8 bit of lock->cnts,
> > no need to subtract the reader count unit specifically. So remove
> > that subtraction to make it clearer, rspin_until_writer_unlock
> > just takes the actual lock->cnts as the 2nd argument.
> >
> > And also change the code comment in queue_write_lock_slowpath to
> > make it more exact and explicit.
> >
> > Signed-off-by: Baoquan He <bhe@redhat.com>
> > ---
> > kernel/locking/qrwlock.c | 8 ++++----
> > 1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> > index f956ede..ae66c10 100644
> > --- a/kernel/locking/qrwlock.c
> > +++ b/kernel/locking/qrwlock.c
> > @@ -76,7 +76,7 @@ void queue_read_lock_slowpath(struct qrwlock *lock)
> > while (atomic_read(&lock->cnts) & _QW_WMASK)
> > cpu_relax_lowlatency();
> >
> > - cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
> > + cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
> > rspin_until_writer_unlock(lock, cnts);
>
> Did you actually look at the ASM generated? I suspect your change makes
> it bigger.


It does make it bigger. But it doesn't matter. Because in
rspin_until_writer_unlock it only compqre (cnts & _QW_WMASK)
with _QW_LOCKED. So using incremented reader count doesn't impact
the result. Anyway it will get the actual lock->cnts in
rspin_until_writer_unlock in next loop. I can't see why we need
subtract that reader count increment specifically.

When I read this code, thought there's some special usage. Finally I
realized it doesn't have special usage, and doesn't have to do that.


\
 
 \ /
  Last update: 2014-12-16 17:21    [W:0.058 / U:0.160 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site