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. 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.
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.
Alan Stern
-
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/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
