Re: frequent lockups in 3.18rc4

[Peter Zijlstra]

On Thu, Dec 18, 2014 at 08:48:24PM -0800, Linus Torvalds wrote:
> On Thu, Dec 18, 2014 at 8:03 PM, Dave Jones <davej@xxxxxxxxxx> wrote:
> >
> > So the only thing that was on that could cause spinlock overhead
> > was DEBUG_SPINLOCK (and LOCK_STAT, though iirc that's not huge either)
> 
> So DEBUG_SPINLOCK does have one big downside if I recall correctly -
> the debugging spinlocks are very much not fair. So they don't work
> like the real ticket spinlocks. That might have serious effects on the
> contention case, with some thread not making any progress due to just
> the implementation of the debug spinlocks.
> 
> Peter, Ingo, maybe I'm full of crap on the debug spinlock thing., but
> a quick look tells me thay are all built on top of the "trylock"
> primitive that does indeed not queue anything, and is thus not fair.
> 
> I'm not sure why the debug spinlocks couldn't just be ticket locks
> instead. But there you are - once more, the debug infrastructure is
> actually much weaker and inferior to the "real" code.

Yeah, the DEBUG_SPINLOCK stuff is horrible. The trylock loops were
designed to 'detect' actual lockups, but this was all done before
lockdep.

I think one can make an argument to remove the trylock loops and fully
rely on lockdep to detect such issues. Only keeping the integrity
checks; similar to the mutex debugging stuff.

There's a related issue with the trylock loops in that it relies on
delay(1) and DVFS heavy (or virt) platforms often have 'dubious' quality
delay loops.
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