On Fri, 2011-03-25 at 07:53 +0100, Tejun Heo wrote: > Hello, Steven, Linus. > > On Thu, Mar 24, 2011 at 09:38:58PM -0700, Linus Torvalds wrote: > > On Thu, Mar 24, 2011 at 8:39 PM, Steven Rostedt <rost...@goodmis.org> wrote: > > > > > > But now, mutex_trylock(B) becomes a spinner too, and since the B's owner > > > is running (spinning on A) it will spin as well waiting for A's owner to > > > release it. Unfortunately, A's owner is also spinning waiting for B to > > > release it. > > > > > > If both A and B's owners are real time tasks, then boom! deadlock. > > > > Hmm. I think you're right. And it looks pretty fundamental - I don't > > see any reasonable approach to avoid it. > > Hmmm... I have an idea. Will play with it a bit and post if it works > out okay.
One solution is to have this be only done on explicit trylocks. Perhaps introduce a mutex_trylock_spin()? Then when the developer knows that this scenario does not exist, they can convert mutex_trylocks() into this spinning version. > > > I think the RT issue is a red herring too - afaik, you can get a > > deadlock with two perfectly normal processes too. Of course, for > > non-RT tasks, any other process will eventually disturb the situation > > and you'd get kicked out due to need_resched(), but even that might be > > avoided for a long time if there are other CPU's - leading to tons of > > wasted CPU time. > > Yeap, need_resched() currently is the only thing which limits the > duration of spinning when the owner continues to run. Yeah, I was about to complain about the long latencies that this could cause, then I realized that RT tasks would in fact deadlock the system here, which I thought was a bigger problem, and focused on that issue. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
