On Mon, Sep 09, 2013 at 11:39:05AM -0400, Steven Rostedt wrote: > On Mon, 9 Sep 2013 11:20:57 -0400 > Steven Rostedt <rost...@goodmis.org> wrote: > > > > It's a bit the same with spinlocks. spinlocks aren't a task > > > synchronization > > > but a CPU synchronization. It's low level. Of course a task can't sleep > > > with a spinlock held (not talking about -rt) so it could be defined as a > > > per > > > task property. But it's just not relevant. > > > > Again, this is where we get into trouble. No it is not a CPU > > synchronization. We only disable preemption because of implementation > > details. Not the concept. A spin lock is only used to protect critical > > data, and not to disable preemption. Those that use it to disable > > preemption has caused us issues in -rt. > > > > This is again the problem with confusing implementation with concepts. > > -rt proved that a spin lock has nothing to do with cpu state, nor > > preemption. > > > > Let me expand on this. Note, using a implementation detail from a item > is known as a side effect, and is frowned on when doing so. > > In fact, when spin_locks() were created, it was just to point out where > critical sections are that prevent more than one task from accessing > some data at the same time. This was needed for multiple CPUs. This was > done before CONFIG_PREEMPT was even created. > > Then Robert Love built on that concept where these same locations > had a characteristic that showed where two tasks can not access the > same data, and used that as preemption points. Points where we can not > be preempted, and let the kernel become preemptible. > > Then -rt built further on the concept, and made these locations able to > sleep by removing the areas that could not sleep before (by threading > IRQs). > > Again, the concept of a spin lock is not about the CPU or even the > task. It is about accessing some data in a safe way. When we stick to > concepts, we can expand on them as we did with CONFIG_PREEMPT and > CONFIG_PREEMPT_RT. It's when people use side effects (disabled > preemption) that breaks this expansion (like those that use spin_locks > and access per_cpu data).
Well, I was considering strict basic spinlocks, sticking to the name. Of course sleeping spinlocks involve the scheduler and the concept of "tasks", and as such complicate the debate :) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
