On Fri, Mar 21, 2014 at 11:24:16AM +0000, Srivatsa S. Bhat wrote: > On 03/21/2014 04:35 PM, Catalin Marinas wrote: > > On Fri, Mar 21, 2014 at 09:21:02AM +0000, Viresh Kumar wrote: > >> @Catalin: We have a problem here and need your expert advice. After changing > >> CPU frequency we need to call this code: > >> > >> cpufreq_notify_post_transition(); > >> policy->transition_ongoing = false; > >> > >> And the sequence must be like this only. Is this guaranteed without any > >> memory barriers? cpufreq_notify_post_transition() isn't touching > >> transition_ongoing at all.. > > > > The above sequence doesn't say much. As rmk said, the compiler wouldn't > > reorder the transition_ongoing write before the function call. I think > > most architectures (not sure about Alpha) don't do speculative stores, > > so hardware wouldn't reorder them either. However, other stores inside > > the cpufreq_notify_post_transition() could be reordered after > > transition_ongoing store. The same for memory accesses after the > > transition_ongoing update, they could be reordered before. > > > > So what we actually need to know is what are the other relevant memory > > accesses that require strict ordering with transition_ongoing. > > Hmm.. The thing is, _everything_ inside the post_transition() function > should complete before writing to transition_ongoing. Because, setting the > flag to 'false' indicates the end of the critical section, and the next > contending task can enter the critical section. smp_mb() is all about relative ordering. So if you want memory accesses in post_transition() to be visible to other observers before transition_ongoing = false, you also need to make sure that the readers of transition_ongoing have a barrier before subsequent memory accesses. > > What I find strange in your patch is that > > cpufreq_freq_transition_begin() uses spinlocks around transition_ongoing > > update but cpufreq_freq_transition_end() doesn't. > > The reason is that, by the time we drop the spinlock, we would have set > the transition_ongoing flag to true, which prevents any other task from > entering the critical section. Hence, when we call the _end() function, > we are 100% sure that only one task is executing it. Hence locks are not > necessary around that second update. In fact, that very update marks the > end of the critical section (which acts much like a spin_unlock(&lock) > in a "regular" critical section). OK, I start to get it. Is there a risk of missing a wake_up event? E.g. one thread waking up earlier, noticing that transition is in progress and waiting indefinitely? -- Catalin -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
