Messages in this thread | | | Date | Sat, 18 Nov 2006 17:13:16 -0500 (EST) | From | Alan Stern <> | Subject | Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync |
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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
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