On Thu, Mar 10, 2011 at 09:48:03AM +0100, Arnd Bergmann wrote: > On Wednesday 09 March 2011 15:36:51 Alexander Shishkin wrote: > > This is another attempt to approach notifying userspace about system > > clock changes. The other one is using an eventfd and a syscall [1]. In > > the course of discussing the necessity of a syscall for this kind of > > notifications, it was suggested that this functionality can be achieved > > via timers [2] (and timerfd in particular [3]). This idea got quite > > some support [4], [5], [6] and some vague criticism [7], so I decided > > to try and go a bit further with it. > > I don't understand from your description or from the patch how user > space gets notified. From your description, I would have expected > to see a change to the timerfd_poll() function to check if the > clock has changed, possibly returning POLLPRI, but the only such > change I can see is in the timerfd_read() function. Don't you need > to change poll() so that a task knows when to call read()? Luckily, no changes for the _poll function were required, because the notification code reuses the ctx->ticks counter of timerfds. IOW, poll wakes up in the same way as before. > > +/* TFD_NOTIFY_CLOCK_SET timers go here */ > > +static DEFINE_SPINLOCK(notifiers_lock); > > +static LIST_HEAD(notifiers_list); > > Maybe I was the only one to be confused by this, but I think t > he naming is slightly misleading, because it's easy to confuse > with a struct notifier_block. You could of course use the > notifier API instead of building your own, but if you don't, > I'd recommend coming up with a different name. Point taken. > I also think that a mutex would be better here than a spinlock. > It's unlikely to be under heavy contention, but if you have > a lot of threads, it could be held for a significant amount > of time. Indeed, thanks! Regards, -- Alex -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
