| Message ID | 1464164289-6124-6-git-send-email-xinhui.pan@linux.vnet.ibm.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui wrote: > cmpxchg_release is light-wight than cmpxchg, we can gain a better > performace then. On some arch like ppc, barrier impact the performace > too much. > > Suggested-by: Boqun Feng <boqun.feng@gmail.com> > Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com> > --- > kernel/locking/qspinlock_paravirt.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h > index a5b1248..2bbffe4 100644 > --- a/kernel/locking/qspinlock_paravirt.h > +++ b/kernel/locking/qspinlock_paravirt.h > @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock) > * unhash. Otherwise it would be possible to have multiple @lock > * entries, which would be BAD. > */ > - locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0); > + locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0); > if (likely(locked == _Q_LOCKED_VAL)) > return; This patch fails to explain _why_ it can be relaxed. And seeing how this cmpxchg() can actually unlock the lock, I don't see how this can possibly be correct. Maybe cmpxchg_release(), but relaxed seems very wrong.
On Thu, May 26, 2016 at 06:47:29PM +0200, Peter Zijlstra wrote: > On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui wrote: > > cmpxchg_release is light-wight than cmpxchg, we can gain a better > > performace then. On some arch like ppc, barrier impact the performace > > too much. > > > > Suggested-by: Boqun Feng <boqun.feng@gmail.com> > > Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com> > > --- > > kernel/locking/qspinlock_paravirt.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h > > index a5b1248..2bbffe4 100644 > > --- a/kernel/locking/qspinlock_paravirt.h > > +++ b/kernel/locking/qspinlock_paravirt.h > > @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock) > > * unhash. Otherwise it would be possible to have multiple @lock > > * entries, which would be BAD. > > */ > > - locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0); > > + locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0); > > if (likely(locked == _Q_LOCKED_VAL)) > > return; > > This patch fails to explain _why_ it can be relaxed. > > And seeing how this cmpxchg() can actually unlock the lock, I don't see > how this can possibly be correct. Maybe cmpxchg_release(), but relaxed > seems very wrong. Clearly I need to stop working for the day, I cannea read. You're doing release, not relaxed. Still Changelog needs improvement.
On 2016年05月27日 00:57, Peter Zijlstra wrote: > On Thu, May 26, 2016 at 06:47:29PM +0200, Peter Zijlstra wrote: >> On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui wrote: >>> cmpxchg_release is light-wight than cmpxchg, we can gain a better >>> performace then. On some arch like ppc, barrier impact the performace >>> too much. >>> >>> Suggested-by: Boqun Feng <boqun.feng@gmail.com> >>> Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com> >>> --- >>> kernel/locking/qspinlock_paravirt.h | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h >>> index a5b1248..2bbffe4 100644 >>> --- a/kernel/locking/qspinlock_paravirt.h >>> +++ b/kernel/locking/qspinlock_paravirt.h >>> @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock) >>> * unhash. Otherwise it would be possible to have multiple @lock >>> * entries, which would be BAD. >>> */ >>> - locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0); >>> + locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0); >>> if (likely(locked == _Q_LOCKED_VAL)) >>> return; >> >> This patch fails to explain _why_ it can be relaxed. >> >> And seeing how this cmpxchg() can actually unlock the lock, I don't see >> how this can possibly be correct. Maybe cmpxchg_release(), but relaxed >> seems very wrong. > > Clearly I need to stop working for the day, I cannea read. You're doing > release, not relaxed. > Never mind. thanks for review :) > Still Changelog needs improvement. > Will do that. thanks xinhui
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h index a5b1248..2bbffe4 100644 --- a/kernel/locking/qspinlock_paravirt.h +++ b/kernel/locking/qspinlock_paravirt.h @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock) * unhash. Otherwise it would be possible to have multiple @lock * entries, which would be BAD. */ - locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0); + locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0); if (likely(locked == _Q_LOCKED_VAL)) return;
cmpxchg_release is light-wight than cmpxchg, we can gain a better performace then. On some arch like ppc, barrier impact the performace too much. Suggested-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com> --- kernel/locking/qspinlock_paravirt.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)