On 11/18, Alan Stern wrote: > > On Sun, 19 Nov 2006, Oleg Nesterov wrote: > > > On 11/18, Alan Stern wrote: > > > > > > By the way, I think the fastpath for synchronize_srcu() should be safe, > > > now that you have added the memory barriers into srcu_read_lock() and > > > srcu_read_unlock(). You might as well try putting it in. > > > > I still think the fastpath should do mb() unconditionally to be correct. > > Yes, it definitely should. > > > > Although now that I look at it again, you have forgotten to put smp_mb() > > > after the atomic_inc() call and before the atomic_dec(). > > > > As I see it, currently we don't need this barrier because synchronize_srcu() > > does synchronize_sched() before reading ->hardluckref. > > > > But if we add the fastpath into synchronize_srcu() then yes, we need mb() > > after atomic_inc(). > > > > Unless I totally confused :) > > Put it this way: If the missing memory barrier in srcu_read_lock() after > the atomic_inc call isn't needed, then neither is the existing memory > barrier after the per-cpu counter gets incremented.
I disagree. There is another reason for mb() after the per-cpu counter gets incremented. Without this barrier we can read the updated value of ->completed (incremented by synchronize_srcu()), but then read a stale data of the rcu protected memory. > Likewise, if a memory > barrier isn't needed before the atomic_dec in srcu_read_unlock(), then > neither is the memory barrier before the per-cpu counter gets decremented. Yes, you are right, I forgot about unlock(), it definitely needs mb(). > What you're ignoring is the synchronize_sched() call at the end of > synchronize_srcu(), which has been replaced with smp_mb(). The smp_mb() > needs to pair against a memory barrier on the read side, and that memory > barrier has to occur after srcu_read_lock() has incremented the counter > and before the read-side critical section begins. Otherwise code in the > critical section might leak out to before the counter is incremented. Still I am not sure you are right. It is ok (I think) if the code in the critical section leaks out to before the atomic_inc(). In fact this doesn't differ from the case when srcu_read_lock() happens before synchronize_srcu() starts. In that case synchronize_srcu() will wait until the critical section is closed via srcu_read_unlock(). Because of synchronize_sched() synchronize_srcu() can't miss the fact that the critical section is in progress, so it doesn't matter if it leaks _before_. Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
