On Thu, Jun 02, 2016 at 06:34:25PM +0200, Peter Zijlstra wrote: > On Thu, Jun 02, 2016 at 04:44:24PM +0200, Peter Zijlstra wrote: > > On Thu, Jun 02, 2016 at 10:24:40PM +0800, Boqun Feng wrote: > > > On Thu, Jun 02, 2016 at 01:52:02PM +0200, Peter Zijlstra wrote: > > > About spin_unlock_wait() on ppc, I actually have a fix pending review: > > > > > > http://lkml.kernel.org/r/1461130033-70898-1-git-send-email-boqun.feng@xxxxxxxxx > > > > > that patch fixed a different problem when people want to pair a > > > spin_unlock_wait() with a spin_lock(). > > > > Argh, indeed, and I think qspinlock is still broken there :/ But my poor > > brain is about to give in for the day. > > This 'replaces' commit: > > 54cf809b9512 ("locking,qspinlock: Fix spin_is_locked() and spin_unlock_wait()") > > and seems to still work with the test case from that thread while > getting rid of the extra barriers. > > --- > include/asm-generic/qspinlock.h | 37 +++++++---------------------------- > kernel/locking/qspinlock.c | 43 +++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 50 insertions(+), 30 deletions(-) > > diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h > index 6bd05700d8c9..9e3dff16d5dc 100644 > --- a/include/asm-generic/qspinlock.h > +++ b/include/asm-generic/qspinlock.h > @@ -28,30 +28,13 @@ > */ > static __always_inline int queued_spin_is_locked(struct qspinlock *lock) > { > - /* > - * queued_spin_lock_slowpath() can ACQUIRE the lock before > - * issuing the unordered store that sets _Q_LOCKED_VAL. > - * > - * See both smp_cond_acquire() sites for more detail. > - * > - * This however means that in code like: > - * > - * spin_lock(A) spin_lock(B) > - * spin_unlock_wait(B) spin_is_locked(A) > - * do_something() do_something() > + /* > + * See queued_spin_unlock_wait(). > * > - * Both CPUs can end up running do_something() because the store > - * setting _Q_LOCKED_VAL will pass through the loads in > - * spin_unlock_wait() and/or spin_is_locked(). > - * > - * Avoid this by issuing a full memory barrier between the spin_lock() > - * and the loads in spin_unlock_wait() and spin_is_locked(). > - * > - * Note that regular mutual exclusion doesn't care about this > - * delayed store. > + * Any !0 state indicates it is locked, even if _Q_LOCKED_VAL > + * isn't immediately observable. > */ > - smp_mb(); > - return atomic_read(&lock->val) & _Q_LOCKED_MASK; > + return !!atomic_read(&lock->val); > } I'm failing to keep up here :( The fast-path code in queued_spin_lock is just an atomic_cmpxchg_acquire. If that's built out of LL/SC instructions, then why don't we need a barrier here in queued_spin_is_locked? Or is the decision now that only spin_unlock_wait is required to enforce this ordering? Will -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
